Synthesis and study of structural,thermal, optical and magnetic properties of stabilized Mn1−x Ni x O system (0·01 <x < 0·30)

A novel chemical passivation route is established to obtain microcrystalline solid solutions of binary oxidé system Mn_1−x Ni _x O (0·01 <x < 0·30). During the passivation process, controlled thermal decomposition of manganous oxalate is carried out to obtain pure MnO and its subsequent reaction with NiO in oxygen-free nitrogen resulted in microcrystalline powder of these solid solutions. The powder is thoroughly characterized by various physicochemical techniques such as XRD, DTA/TG/DTG, diffused reflectance spectra, magnetic susceptibility, TEM, XPS etc. The observed processing-structure-property correlations confirmed the improved thermal stability of these powders (relative to pure MnO) in air. The important role of dopant paramagnetic Ni²⁺ ions in enhancing the passivation of the bulk Mn²⁺ species is explained on the basis of the formation of mixed oxide complex species on the surface of these microcrystalline powders.     

Specific contact resistance and metallurgical process of the silver-based paste for making ohmic contact structure on the porous silicon/p-Si surface of the silicon solar cell

Porous silicon has been considered as a promising optoelectronic material for developing a variety of optoelectronic devices and sensors. In the present study, the electrical properties and metallurgical process of the screen-printed Ag metallization formed on the porous silicon surface of the silicon solar cell have been investigated. The contact structure consists of thick-film Ag metal contact patterned on the top of the porous silicon surface. The sintering process consists of a rapid firing step at 750–825 °C in air ambient. It results in the formation of a nearly perfect contact structure between the Ag metal and porous silicon/p-Si structure that forms the top metalization for the screen-printed silicon solar cells. The SEM picture shows that Ag metal firmly coalesces with the silicon surface with a relatively smooth interfacial morphology. This implies that high temperature fire-through step has not introduced any signs of adverse effect of junction puncture or excessive Ag indiffusion, etc. The three-point probe (TPP) method was applied to estimate the specific contact resistance, ρ _c (Ω-cm²) of the contact structure. The TPP measurement shows that contact structure has excellent ohmic properties with ρ _c = 1.2 × 10⁻⁶ Ω-cm² when the metal contact sintered at 825 °C. This value of the specific contact resistance is almost three orders of magnitude lower than the corresponding value of the ρ _c = 7.35 × 10⁻³ Ω-cm² obtained for the contact structure sintered at 750 °C. This improvement in the specific contact resistance indicates that with increase in the sintering temperature, the barrier properties of the contact structure at the interface of the Ag metal and porous silicon structure improved which in turn results a lower specific contact resistance of the contact structure.     

Specific contact resistance measurements of the screen-printed Ag thick film contacts in the silicon solar cells by three-point probe methodology and TLM method

The specific contact resistance of the screen-printed Ag contacts in the silicon solar cells has been investigated by applying two independent test methodologies such as three-point probe (TPP) and well-known transfer length model (TLM) test structure respectively. This paper presents some comparative results obtained with these two measurement techniques for the screen-printed Ag contacts formed on the porous silicon antireflection coating (ARC) in the crystalline silicon solar cells. The contact structure consists of thick-film Ag metal contacts patterned on the top of the etched porous silicon surface. Five different contact formation temperatures ranging from 725 to 825 °C for few minutes in air ambient followed by a short time annealing step at about 450 °C in nitrogen ambient was applied to the test samples in order to study the specific contact resistance of the screen-printed Ag metal contact structure. The specific contact resistance of the Ag metal contacts extracted based on the TPP as well as TLM test methodologies has been compared and verified. It shows that the extraction procedure based on the TPP method results in specific contact resistance, ρ _c  = 2.15 × 10⁻⁶ Ω-cm² indicating that screen-printed Ag contacts has excellent ohmic properties whereas in the case of TLM method, the best value of the specific contact resistance was found to be about ρ _c  = 8.34 × 10⁻⁵ Ω-cm². These results indicate that the ρ _c value extracted for the screen-printed Ag contacts by TPP method is one order of magnitude lower than that of the corresponding value of the ρ _c extracted by TLM method. The advantages and limitations of each of these techniques for quantitatively evaluating the specific contact resistance of the screen-printed Ag contacts are also discussed.     

Preparation and characterization of solid-state sintered aluminum-doped zinc oxide with different alumina contents

Aluminum-doped zinc oxide (AZO) ceramics with 0− 2 ·5 wt.% alumina (Al₂O₃) content were prepared using a solid-state reaction technique. It was found that AZO grains became finer in size and more irregular in shape than undoped ZnO as the Al₂O₃ content increased. Addition of Al₂O₃ dopant caused the formation of phase transformation stacking faults in ZnO grains. The second phase, ZnAl₂O₄ spinel, was observed at the grain boundaries and triple junctions, and inside the grains. In this study, a 3-inch circular Al₂O₃ (2 wt.%)-doped ZnO ceramic target sintered at 1500°C for 6 h has a relative density of 99·8% with a resistivity of 1·8 × 10^− 3 Ω-cm. The AZO film exhibits optical transparency of 90·3% in the visible region and shows an electrical resistivity of 2·5 × 10^− 3 Ω-cm.     

DLTS study of annihilation of oxidation induced deep-level defects in Ni/SiO2/n-Si MOS structures

This paper describes the fabrication of MOS capacitor and DLTS study of annihilation of deeplevel defects upon thermal annealing. Ni/SiO₂/n-Si MOS structures fabricated on n-type Si wafers were investigated for process-induced deep-level defects. The deep-level traps in Si substrates induced during the processing of Ni/SiO₂/n-Si have been investigated using deep-level transient spectroscopy (DLTS). A characteristic deep-level defect at E _C = 0·49 eV which was introduced during high-temperature thermal oxidation process was detected. The trap position was found to shift to different energy levels (E _C = 0·43, 0·46 and 0·34 eV) during thermal annealing process. The deep-level trap completely anneals at 350°C. Significant reduction in trap density with an increase in recombination life time and substrate doping concentration as a function of isochronal annealing were observed.     

Thickness dependent electrical properties of CdO thin films prepared by spray pyrolysis method

A large number of thin films of cadmium oxide have been prepared on glass substrates by spray pyrolysis method. The prepared films have uniform thickness varying from 200–600 nm and good adherence to the glass substrate. A systematic study has been made on the influence of thickness on resistivity, sheet resistance, carrier concentration and mobility of the films. The resistivity, sheet resistance, carrier concentration and mobility values varied from 1·56–5·72×10⁻³ Ω-cm, 128–189 Ω/□, 1·6–3·9×10²¹ cm⁻³ and 0·3–3 cm²/Vs, respectively for varying film thicknesses. A systematic increase in mobility with grain size clearly indicates the reduction of overall scattering of charge carriers at the grain boundaries. The large concentration of charge carriers and low mobility values have been attributed to the presence of Cd as an impurity in CdO microcrystallites. Using the optical transmission data, the band gap was estimated and found to vary from 2·20–2·42 eV. These films have transmittance around 77% and average reflectance is below 2·6% in the spectral range 350–850 nm. The films aren-type and polycrystalline in nature. SEM micrographs of the CdO films were taken and the films exhibit clear grains and grain boundary formation at a substrate temperature as low as 523 K.     

Effects of nanoscale size dependent parameters on lattice thermal conductivity in Si nanowire

The effects of nanoscale size dependent parameters on lattice thermal conductivity are calculated using the Debye-Callaway model including transverse and longitudinal modes explicitly for Si nanowire with diameters of 115, 56, 37 and 22 nm. A direct method is used to calculate the group velocity for different size nanowire from their related calculated melting point. For all diameters considered, the effects of surface roughness, defects and transverse and longitudinal Gruneisen parameters are successfully used to correlate the calculated values of lattice thermal conductivity to that of the reported experimental curve. The obtained fitting value for mean Gruneisen parameter has a systematic dependence on all Si nanowire diameters changing from 0·791 for 115 nm diameter to 1·515 for the 22 nm nanowire diameter. The dependence also gave a suggested surface thickness of about 5–6 nm. The other two parameters were found to have partially systematic dependence for diameters 115, 56, and 37 nm for defects and 56, 37 and 22 nm for the roughness. When the diameters go down from 115 to 22 nm, the concentration of dislocation increased from 1·16 × 10¹⁹cm⁻³ to 5·20 × 10¹⁹cm⁻³ while the surface roughness P found to increase from 0·475 to 0·130 and the rms height deviation from the surface changes by about 1·66 in this range of diameter. The diameter dependence also indicates a strong control of surface effect in surface to bulk ratio for the 22 nm wire diameter.     

A comparison of nitrous oxide and hydrogen ECR plasma discharges for silicon solar cell passivation

The passivation of crystalline Si solar cells using nitrous oxide (N₂O) electron cyclotron resonance (ECR) plasma discharges has been studied and compared with ECR hydrogen passivation. The cells consisted of ECRCVD grown microcrystalline Si emitter layers on single crystal Si (sc-Si) and multicrystalline Si (mc-Si) substrates, without anti-reflective coatings or surface texturing. For cells on sc-Si substrates, hydrogen passivation is more effective at a substrate temperature of 300 °C and low microwave power (300 W). With increased power (500 W) H₂ is less effective than N₂O due to hydrogen plasma damage leading to a significant fall in the cell fill factor. In comparison with H₂, N₂O discharges lead to a significantly better (by > a factor of 2) improvement in the performance of cells on mc-Si substrates for treatment times of ≤15 min at a passivation temperature of 300 °C and 300 W microwave power. XPS measurements suggest that a surface oxide layer containing N and C atoms is formed by the N₂O plasma which, most likely, reduces the surface state density and, hence, carrier recombination.     

Measurements of thermophysical properties of molten silicon by a high-temperature electrostatic levitator

Several thermophysical properties of molten silicon measured by the high-temperature electrostatic levitator at JPL are presented. They are density, constant-pressure specific heat capacity, hemispherical total emissivity, and surface tension. Over the temperature range investigated (1350<T _m<1825 K), the measured liquid density (in g·cm⁻³) can be expressed by a quadratic function,p(T)=p _m−1.69×10⁻⁴(T−T _m)−1.75×10⁻⁷(T−T _m)² withT _m andp _m being 1687 K and 2.56 g·cm⁻³, respectively. The hemispherical total emissivity of molten silicon at the melting temperature was determined to be 0.18, and the constant-pressure specific heat was evaluated as a function of temperature. The surface tension (in 10⁻³ N·m⁻¹) of molten silicon over a similar temperature range can be expressed by σ(T)=875–0.22(T−T _m). Invited paper presented at the Fourth Asian Thermophysical Properties Conference, September 5–8, 1995, Tokyo, Japan.     

Enhanced optical performance by energetic hydrogen passivation at Si/oxide interface

Plasma immersion ion implantation (PIII) of hydrogen can provide appropriate kinetic energy to passivate the Si/SiO₂ interface. To avoid excessive damage of hydrogen, the implantation is performed with low kinetic energy (100 eV). Passivation decreases the dark current and enhances responsivity of metal-oxide-semiconductor tunneling photodetectors. The dependence of photoluminescence (PL) intensity on surface recombination velocity is theoretically studied. The intensity enhancement of PL also indicates that surface recombination velocity at Si/SiO₂ is significantly reduced after the PIII passivation. Since PIII is capable of isotropic implantation, tunable penetration depth, and large area process, it is an ideal tool for Si passivation with high throughput.     

Solution growth of cadmium telluride

Cadmium telluride crystal has been grown by solution method from Te-rich (Cd_0·3Te_0·7) melt. Ingots having 9 mm diameter and length up to 30 mm were grown by cooling the melt slowly (1°C/h) under a vertical temperature gradient of about 30°C/cm. As-grown ingots were characterized for optical transmission and resistivity. The middle portion of the ingots exhibited better optical transmission properties. Resistivity (p-type) was found increasing, towards the last-to-grow end, from 10³ to 10⁶ Θ-cm. Surface barrier type of detectors, made from low resistivity (≅ 10⁴Θ-cm) materials, were found suitable for detection ofX- and low energy gamma radiations. In case of high resistivity (≅10⁶Θ-cm) detectors, the performance was seen to be affected by polarization. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

Nanostructure formation and passivation of large-area black silicon for solar cell applications

Nanoscale textured silicon and its passivation are explored by simple low-cost metal-assisted chemical etching and thermal oxidation, and large-area black silicon was fabricated both on single-crystalline Si and multicrystalline Si for solar cell applications. When the Si surface was etched by HF/AgNO(3) solution for 4 or 5 min, nanopores formed in the Si surface, 50-100 nm in diameter and 200-300 nm deep. The nanoscale textured silicon surface turns into an effective medium with a gradually varying refractive index, which leads to the low reflectivity and black appearance of the samples. Mean reflectance was reduced to as low as 2% for crystalline Si and 4% for multicrystalline Si from 300 to 1000 nm, with no antireflective (AR) coating. A black-etched multicrystalline-Si of 156 mm × 156 mm was used to fabricate a primary solar cell with no surface passivation or AR coating. Its conversion efficiency (η) was 11.5%. The cell conversion efficiency was increased greatly by using surface passivation process, which proved very useful in suppressing excess carrier recombination on the nanostructured surface. Finally, a black m-Si cell with efficiency of 15.8% was achieved by using SiO(2) and SiN(X) bilayer passivation structure, indicating that passivation plays a key role in large-scale manufacture of black silicon solar cells.     

Transparent conducting films

We present here a new technique, called vapour phase spray pyrolysis, for deposition ofto andito films. Undopedto film showed semiconducting nature, a sheet resistance of 1088 Ω/□, a donor ionization energy level of 40 meV, average visible transmittance of 75·4% andn-type conductivity. The indium doped (ito) film 10 wt % showed metallic nature, a sheet resistance of 15 Ω/□, average visible transmittance of 80·4 % andp-type conductivity. Thus theto andito films showed fairly good electro-optical qualities, comparable to those obtained by sophisticated and costly techniques.     

Effect of Ca/Si ratio on the microstructures and properties of CaO–B2O3–SiO2 glass-ceramics

CaO–B₂O₃–SiO₂ glass-ceramics were synthesized by sol–gel method, and the effect of Ca/Si ratio on the microstructures, electrical properties and mechanical characteristics of this ternary system was investigated. The results showed that the increase of CaO content is favorable for the crystallization of CaO–B₂O₃–SiO₂ system and formation of the desired glass-ceramics. The bending strength of the sintered glass-ceramics increases with CaO content by increasing of crystalline phases. When the Ca/Si ratio increases, the dielectric constant (ε _r) decreases and loss (tanδ) increases gradually. The thermal expansion coefficient (TEC) enhances by increasing CaO contents due to the formation of other crystal phases with large TEC value. The glass-ceramics exhibit low dielectric constant and loss (ε _r < 4.7, tanδ < 5 × 10^-4 at 1 MHz), high resistivity (ρ > 10¹² Ω · cm), as well as excellent mechanical properties (σ ≈ 160 MPa, α ≈ 3.6 × 10⁻⁶/°C).     

Studies on relative catalytic efficiencies of zinc and aluminium on nucleation of dispersed bismuth droplets

In Zn-Bi and Al-Bi systems, both of which belong to monotectic class, dispersion of second phase particles within the matrix have been produced through rapid solidification processing (RSP) route. In order to understand relative catalytic efficiencies of Zn and Al matrices in catalyzing nucleation of Bi particles entrapped in the respective metal matrices, thermal analysis in constant program mode was performed. Thermal analyses revealed that Bi undercools by 132° in Zn matrix and by 157° in Al matrix. Thermodynamic barrier to nucleation (ΔG*) for Bi has also been calculated, which is 39·8 kcal/mole and 47·085 kcal/mole, in Zn and Al matrices, respectively at the maximum recorded undercoolings.     

Experimental Reconstruction of Thermal Parameters in CNT Array Multilayer Structure

The thermal conductivities, thermal diffusivity, thermal anisotropy ratio, and thermal boundary resistance for the multilayered microstructure of a carbon nanotube (CNT) array are reconstructed experimentally using the 3ω method with two different width metal heaters. The thermal impedance in the frequency domain and sensitivity coefficients are introduced to simultaneously determine the multiple thermal parameters. The thermal conductivity at 295 K is 38 W · m⁻¹ · K⁻¹ along the nanotube growth direction, and two orders of magnitude lower in the direction perpendicular to the tubes with the anisotropy ratio as large as 86. Separation of the contact and CNT array resistances is realized through circuit modeling. The measured thermal boundary resistances of the CNT array/Si substrate and insulating diamond film interfaces are 3.1 m² · K · MW⁻¹ and 18.4 m² · K · MW⁻¹, respectively. The measured thermal boundary resistance between the heater and diamond film is 0.085 m² · K · MW⁻¹ using a reference sample without a CNT array. The thermal conductivity for a CNT array already exceeds those of phase-changing thermal interface materials used in microelectronics.     

Deep surface states on the interface between SiC and its native thermal oxide

Deep surface states are discovered on the interface between 6H-SiC and its native thermal oxide by analyzing the C-V characteristics of metal-oxide-semiconductor structures measured at a high temperature (600 K). The maximum of the density of states distributed according to energy (D _tm=2×10¹² cm⁻²· eV⁻¹) is at an energy about 1.2 eV below the bottom of the conduction band of SiC. It is postulated that the states discovered are similar in nature to the P _b centers observed in the SiO₂/Si system. Pis’ma Zh. Tekh. Fiz. 23, 55–60 (October 26, 1997)     

A comparison of nitrous oxide and hydrogen ECR plasma discharges for silicon solar cell passivation

The passivation of crystalline Si solar cells using nitrous oxide (N₂O) electron cyclotron resonance (ECR) plasma discharges has been studied and compared with ECR hydrogen passivation. The cells consisted of ECRCVD grown microcrystalline Si emitter layers on single crystal Si (sc-Si) and multicrystalline Si (mc-Si) substrates, without anti-reflective coatings or surface texturing. For cells on sc-Si substrates, hydrogen passivation is more effective at a substrate temperature of 300 °C and low microwave power (300 W). With increased power (500 W) H₂ is less effective than N₂O due to hydrogen plasma damage leading to a significant fall in the cell fill factor. In comparison with H₂, N₂O discharges lead to a significantly better (by > a factor of 2) improvement in the performance of cells on mc-Si substrates for treatment times of ≤15 min at a passivation temperature of 300 °C and 300 W microwave power. XPS measurements suggest that a surface oxide layer containing N and C atoms is formed by the N₂O plasma which, most likely, reduces the surface state density and, hence, carrier recombination.     

Electron structure and activation energy of hydrogen in α-Zr using nonlinear response theory

The electron structure of hydrogen in hcp Zr is calculated by using self-consistent nonlinear screening theory. The host-ion contribution is included through the spherical solid model potential (SSMP). The resulting charge density and scattering phase shifts are used to calculate the activation energy and residual resistivity of hydrogen in α-Zr matrix. The calculated activation energy 0·285 eV is found in reasonably good agreement with experimental value 0·3 eV. The estimated residual resistivity 0·53 μΩ cm/at% for Zr-H system using the scattering phase shifts agrees reasonably well with the observed value 0·27 μΩ cm/at%. The calculated configurational energy shows that hydrogen prefers tetrahedral(T)-sites over octahedral(O)-sites in α-Zr. The strong binding energy of electron-proton suggests that hydrogen forms zirconium hydride.     

Synthesis and characterization of high surface area molybdenum nitride

The synthesis of high surface area γ-Mo₂N materials using the nitridation of oxide precursors MoO₃, H₂MoO₅, and H₂MoO₅·H₂O with ammonia at 650°C is described. H₂MoO₅ and its hydrated form were obtained from the reaction of MoO₃ and diluted H₂O₂. The materials were characterized by means of X-ray powder diffraction, thermal analysis and nitrogen physisorption. Directly after the preparation, the nitride materials were subjected to different processing conditions: (1) contact to air, (2) inert gas or (3) treated with 1% O₂(g)/N₂(g) gas mixture (Passivation). The synthesis and passivation conditions critically affect the specific surface area of the final product. By means of XRD a minor quantity of MoO₂ was detected in most of the products. The highest specific surface area of the nitrides was 158.4 m²/g for γ-Mo₂N materials using H₂MoO₅·H₂O as the precursor. The high specific surface area corresponds to an average particle diameter of 4 nm, assuming a cubic morphology of the nanocrystals (d_p = 6/ρS_BET, ρ = 9.5 g/cc). The nitrogen physisorption isotherms of γ-Mo₂N are of type IV, but pore sizes and diameters differ significantly depending on the synthesis conditions due to different defect structures of the intermediates generated in the course of the topotactic transformation of the oxides to nitrides.