H.K. Sehgal

Structure, conductivity and Hall effect study of solution grown Pb1?xFexS nanoparticle films

Nanoparticle films of ternary Pb1?xFexS alloys were grown on glass and quartz substrates using a chemical bath deposition technique. Critical control of pH, temperature and dilution of the chemical bath was necessary for growth of the nanoparticle films. The ternary films were observed to grow as single phase with a lattice identical to that of PbS. Analysis of temperature-dependent dc conductivity data suggests variable-range hopping to be the conduction mechanism. Room-temperature Hall mobility of the films is found to increase with an increase in x and has a temperature dependence of ? T?0.4, which suggests predominant surface scattering. Majority carriers were found to be holes in the 0.25 ? x ? 0.50 films and electrons in the x = 0.66 and 0.75 films.

Structure of flash-evaporated Pb1−xHgxS alloy films

Abstract Electron diffraction and differential thermal analysis investigations were carried out on films prepared by the flash evaporation of PbS-HgS mixtures on freshly cleaved single crystals of KBr. The results suggest that under appropriate growth conditions PbS alloys with α-HgS to form Pb 1− x Hg x S over the composition range 0.07 ⩽ χ ⩽0.93. Lead-rich films (0.07 ⩽ χ ⩽0.50) deposited at a substrate temperature T of 100°C had f.c.c. structures and grew epitaxially with 〈100〉 grain orientation. Depositions at lower substrate temperatures resulted in polycrystalline films with distorted f.c.c. lattices; the lattice parameters increased with increasing mercury concentration. The mercury-rich films (0.60 ⩽ χ ⩽0.93) grown at T = 25°C were amorphous. An increase in T resulted in a change of the amorphous structure through polycrystalline simple cubic at T = 50°C to distorted f.c.c. at T = 100°C.

Chemically Deposited PbSe Nanoparticle Films Variation in Shape and Size

PbSe nanocrystalline thin films have been prepared on glass and quartz substrates by chemical bath deposition method, using sodium selenosulfate, which acts both as a source of Se 2- ions and a complexing agent, in the chemical bath. Effect of pH and temperature variation of the bath on grain size in the films is investigated; it is observed that the grain size increases with increase in pH and temperature. The films grow with typical rock salt structure of PbSe and the lattice parameter shows a slight increase with decrease in the grain size. Morphology of the grains is observed to be sensitive to their size. While the smaller grains are spherical, the larger nanoparticles appear to gradually transform from spherical to square platelets and cubes. Optical bandgaps (Eg) in the nanocrystalline films increase with decrease in grain size and have values larger than 0.28 eV of bulk PbSe. The observed blueshift in the films is expected to originate from quantum confinement in the nanograins.

Study of Solution Grown Variable BandGap Pb1 − x Mn x S Semiconductor Nanoparticle Films

Single-phase Pb 1-x Mn x S (0.03 ≤ x ≤ 0.37) nanoparticle films were prepared using an electroless solution growth technique. Face centered cubic structure with a decrease of lattice parameter for an increase of x in the alloy films was observed. Composition dependence of optical and electrical properties is discussed. The optical bandgap (E g ) could be varied from 1.50 to 2.50 eV by changing the x in the range 0.03 ≤ x ≤ 0.37 for the films. Effect of the grain size and the effect of alloying on optical properties are distinguished for the ternary nanoparticle films. Variable range hopping has been identified as mechanism for conduction in the p-type semiconductor Pb 1-x Mn x S nanoparticle films. Mobility (μ of the majority carriers shows a decrease with increase in Mn concentration x and follows a T -0.4 temperature dependence, indicating the surface scattering of carriers in the nanoparticle films.

Bias-induced changes in carrier type of Pb(1−x)Fe(x)S nanocrystalline solution grown thin films

Change of majority carrier type was achieved in nanocrystalline Pb(1−x)Fe(x)S thin films by applying a DC-bias on the substrate during their growth by solution growth technique. Critical control of biasing and pH of the solution bath are observed to be necessary to achieve this change. The DC-bias results in slight change in relative concentrations of the constituents in the films, which lead to change in majority carriers without any measurable change in lattice parameter. This modification to the conventional solution growth technique provides the possibility to grow abrupt homojunctions in the nanocrystalline films by sudden change in DC-biasing during film growth.

Optical and electrical properties of flash-evaporated Pb1−xHgxSe films

Abstract Results of optical and electrical properties of flash-evaporated Pb 1− x Hg x Se films, in the composition range 0.07 ⩽ x ⩽ 0.93, show that the alloy films are formed by alloying of PbSe and HgSe. The optical band gaps of films grown on substrates at 25 and 100°C decrease linearly from 0.20 to 0.09 eV and from 0.15 to 0.065 eV, respectively, as the Hg concentration in the films is increased from 0.07 to 0.93. Activation energy values, as obtained from conductivity measurements, suggest that between 225 and 400 K the conduction results mainly from thermally-generated charge carriers, whereas at the lower temperatures ( 400 K.

The effect of DC biasing of Si substrates on the structure and grain size of Pb1?xFexS nanoparticle films

Pb1−xFexS nanoparticle films were grown on silicon substrates under zero, positive and negative biasing by the chemical bath deposition technique. The grain size for films with a fixed x was observed to increase with positive biasing and decrease with negative biasing of the substrates. Distortion of the lattice is observed to increase and the average grain size decreases as the substrate bias is changed from positive through zero to negative values. The DC conductivity of the films decreases with decrease in grain size due to enhanced surface scattering. The nanoparticle films are semiconducting and show a predominance of variable range hopping of the carriers among localized states.

Properties of variable band gap II-VI-IV ternary alloy thin films

Abstract Thin films of (PbCH)1-x-(HgCH)x (CH=S, Se and Te; 0.07⩽x⩽0.93) materials were deposited at different temperatures by flash evaporation. Structural and optical analyses indicate that under appropriate conditions of growth, the films grow as single phase and the characteristics are sensitive to mercury concentration (x) and substrate temperature. The direct optical band gap can be varied between 1.43–2.17 eV in Pb1-xHgxS, 0.20-0.06 eV in Pb1-xHgxSe and 0.27-0.09 eV in Pb1-xHg xTe films by changing mercury concentration from 0.07 to 0.93. The observed characteristics of the films can be explained by considering that the new lattices form by atom-by-atom condensation of evaporated materials.

Composition dependence of optical and electrical characteristics in flash evaporated Pb1−xHgxTe films

Structural, optical and electrical analyses of flash evaporated lead-mercury telluride films indicate that under appropriate conditions of growth ternary Pb1−xHgxTe compounds are formed in the single phase crystalline films. Optical band gaps of the crystalline films, grown on substrates at temperatures (Ts) of 25 and 100°C, are observed to decrease with increase in mercury concentration in the films. The optical gap decreases between 0.27 and 0.14 eV in the 0.07 ⩽x⩽ 0.50 Ts of 25°C films and between 0.23 and 0.09 eV in the Ts of 100°C 0.07 ⩽x⩽ 0.93 films. Activation energy values, calculated from electrical conductivity measurements, indicate that between 350 and 500 K, the conduction is mainly due to thermally generated charge carriers, whereas impurity conduction dominates at temperatures <250K. All crystalline films are observed to be photoconducting.