D. Kumar

Luminescence of pulsed laser deposited Eu doped yttrium oxide films

Europium doped yttrium oxide (Eu:Y2O3) phosphor thin films were grown using a pulsed laser deposition (PLD) technique at varying growth conditions. The structural characterization carried out on a series of Eu:Y2O3 films grown on (100) silicon at substrate temperatures in the range of 250–600 °C and oxygen pressure in the range of 10−5 Torr to 200 mTorr indicated that films were preferentially (111) oriented. Measurements of photoluminescence and cathodoluminescence properties of laser deposited Eu:Y2O3 thin films and powder used for laser target showed that the best in situ grown films were ∼10%–22% as bright as Eu:Y2O3 powder. A postdeposition annealing treatment of Eu:Y2O3 films led to further improvements in their brightness (up to ∼70% with respect to Eu:Y2O3 powder), with cluster sizes of <400 nm.

Improved luminescence properties of pulsed laser deposited Eu:Y2O3thin films on diamond coated silicon substrates

Europium activated yttrium oxide (Eu:Y2O3) phosphor films have been grown in situ on (100) bare and diamond-coated silicon substrates using a pulsed laser deposition technique. Diamond-coated silicon substrates were prepared using hot filament chemical vapor deposition of diamond onto silicon. Photoluminescence brightness from Eu:Y2O3 films grown at 700 °C on diamond-coated silicon substrates was about twice that of films on bare silicon, and reached 80% of the brightness of powders. The higher brightness from Eu:Y2O3 film on diamond-coated silicon substrates is attributed to reduced internal reflections from the Eu:Y2O3 film surface, which results from the roughness of the diamond layer.

Ferromagnetism in Cu-doped ZnO films: Role of charge carriers

We report the observation of room temperature ferromagnetism in Cu-doped (5%) ZnO films grown on c-plane sapphire substrates. Films were prepared by pulsed laser deposition technique and were thoroughly characterized using several state-of-the-art characterization techniques. Hall measurements showed that the films are of n-type with a carrier concentration of 3×1017cm−3. Magnetization measurements showed that the films exhibit room temperature ferromagnetism with a saturation magnetization of ∼1.45μB∕Cu atom. When additional carriers were introduced in the films, ferromagnetism was completely vanished. Our results show that the p-type nature of the film is not essential for realizing ferromagnetic characteristics; however, the concentration of n-type carriers should not exceed a critical value.

Rectifying electrical characteristics of La0.7Sr0.3MnO3/ZnO heterostructure

We have fabricated a p–n junction, consisting of p-type manganite (La0.7Sr0.3MnO3) and n-type ZnO layers grown on sapphire substrate. This junction exhibits excellent rectifying behavior over the temperature range 20–300 K. Electrical characteristics of La0.7Sr0.3MnO3 (LSMO) film in this heterostructure are found to be strongly modified by the built-in electric field at the junction. It has been shown that by applying the external bias voltage, the thickness of the depletion layer, and hence, the electrical and magnetic characteristics of LSMO film can precisely be modified.We have fabricated a p–n junction, consisting of p-type manganite (La0.7Sr0.3MnO3) and n-type ZnO layers grown on sapphire substrate. This junction exhibits excellent rectifying behavior over the temperature range 20–300 K. Electrical characteristics of La0.7Sr0.3MnO3 (LSMO) film in this heterostructure are found to be strongly modified by the built-in electric field at the junction. It has been shown that by applying the external bias voltage, the thickness of the depletion layer, and hence, the electrical and magnetic characteristics of LSMO film can precisely be modified.

Ferromagnetism in Ni-doped ZnO films: Extrinsic or intrinsic?

Here we report a detailed study aimed on understanding the origin of ferromagnetism in Ni-doped ZnO films. A pulsed laser deposition technique was used to deposit Ni-doped (5 at. %) ZnO films on sapphire (0001) substrates under different oxygen pressures ranging from 10−6 to 0.1 Torr. Films were characterized using numerous characterization techniques including x-ray diffraction, x-ray photospectroscopy, energy dispersive x-ray spectroscopy, optical absorption spectroscopy, and electrical transport, magnetotransport, and magnetization measurements. A detailed structure-property correlation and analysis of our results revealed that the ferromagnetism in ZnO:Ni films is not an inherent property of the material but results due to a strong tendency of Ni to precipitate out in the ZnO matrix.

Pulsed laser deposition and characterization of high-Tc YBa2Cu3O7 − x superconducting thin films

Abstract This article presents a systematic overview of the work carried out in the area of pulsed laser deposition (PLD), characterization and device application aspects of high temperature superconducting YBCO thin films. The theoretical and experimental aspects of the pulsed laser deposition process for the synthesis of YBCO thin films have been described in detail. The deposition technique has emerged as a very powerful method to make composition and microstructure controlled superconducting YBCO films. Some unique features of this process are due to the rapid heating and evaporation of the target and the interaction of the laser beam with the evaporated materials leading to the formation of a high temperature plasma. Major advantages of PLD process are congruent evaporation and crystallinity due to the presence of high energy evaporants and fast response time. It has the potential to encompass a wide scope of physical vapor-deposition techniques from thermal evaporation to sputtering and MBE. The electrical and structural studies performed on laser deposited YBCO films have shown that films produced by PLD are superior than films produced by other thin film growth techniques. The major technical obstacle of particulates emission, encountered initially with PLD, has now greatly been solved. Some new developments with PLD technology such as large-area and nonplanar substrates coatings and growth of novel superconductors as oriented films have been demonstrated.

Enhancement of flux pinning in YBa2Cu3O7−δ thin films embedded with epitaxially grown Y2O3 nanostructures using a multi-layering process

Nanodot arrays of Y2O3 were dispersed in thin films of YBa2Cu3O7−δ (YBCO) by growing alternating layers of these two species using a pulsed laser deposition method. As a result, critical current density Jc both in applied magnetic field and self-field is enhanced by as much as an order of magnitude, along with a significant increase in the irreversibility field Hirr. High-resolution scanning transmission electron microscopy (STEM) and Z -contrast STEM show that the nanoparticles are crystalline and coherent with the YBCO matrix. Whereas in most other studies pinning has been attributed to the strain fields around the nanoparticles, in this case pinning may actually be due to the nanoparticles themselves ,s incethe delineation between the two species is very sharp and STEM reveals no discernible strain fields in the superconducting material around the nanoparticles. (Some figures in this article are in colour only in the electronic version)

Growth and Characterization of Eu:Y[sub 2]O[sub 3] Thin-Film Phosphors on Silicon and Diamond-Coated Silicon Substrates

Europium-activated yttrium oxide (Eu:Y{sub 2}O{sub 3}) phosphor films have been grown in situ on (100) bare silicon and diamond-coated silicon substrates using a pulsed laser deposition technique. Diamond-coated silicon substrates were prepared by hot filament chemical vapor deposition onto (100) silicon wafers. Measurements of photoluminescence and cathodoluminescence properties of Eu:Y{sub 2}O{sub 3} films have shown that the films grown on diamond-coated silicon substrates are brighter than the films grown on bare silicon substrates under identical deposition conditions. The improved brightness of the Eu:Y{sub 2}O{sub 3} films on diamond-coated silicon substrates is attributed to reduced internal reflection, low photon energy absorption by substrate, and enhanced scattering of incident beam with lattice. All these effects are primarily brought about by the presence of a rough diamond interfacial layer between the phosphor films and substrates.

Epitaxial growth of ZnO films on Si(111)

In this paper, we report the growth of ZnO films on silicon substrates using a pulsed laser deposition technique. These films were deposited on Si(111) directly as well as by using thin buffer layers of AlN and GaN. All the films were found to have c-axis-preferred orientation aligned with normal to the substrate. Films with AlN and GaN buffer layers were epitaxial with preferred in-plane orientation, while those directly grown on Si(111) were found to have random in-plane orientation. A decrease in the frequency of the E 2 (2) Raman mode and a red shift of the band-edge photoluminescence peak due to the presence of tensile strain in the film, was observed. Various possible sources for the observed biaxial strain are discussed.

High coercivity and superparamagnetic behavior of nanocrystalline iron particles in alumina matrix

Single-domain nanoscale magnetic iron particles have been embedded uniformly in an amorphous matrix of alumina using a pulsed laser deposition technique. Structural characterization by transmission electron microscopy (TEM) reveals the presence of a crystalline iron and an amorphous alumina phase. Fine particle magnetism have been investigated by carrying out field and temperature dependence of magnetization measurements using superconducting quantum interference device magnetometer. The particle size of Fe in Al2O3 matrices prepared by changing the deposition time of Fe, have been found to be 9, 7 and 5 nm from TEM studies. At 10 K, the coercivities of these samples are found be 450, 350 and 150 Oe, respectively. At 300 K, the coercivity of Fe–Al2O3 sample decreases from 100 to 50 Oe as the particle size decreases from 9 to 7 nm and finally the sample turns superparamagnetic when the Fe particle size becomes around 5 nm. Based on the calculated value of blocking temperature, TB, (481 K), magnetic anisotropy K (4.8 � 10 5 erg/cm 3 ) for Fe, and the Boltzmann constant kB (1.38 � 10 @16 erg/K) from TB=KV/25kB, the mean radius of Fe particles is found to be 9.3 nm. in one of the samples. This is in good agreement with the particle size measured using TEM studies. r 2001 Published by Elsevier Science B.V.