S. Sharan

In situ single chamber laser processing of YBa2Cu3O7−δ superconducting thin films on Si (100) with yttria‐stabilized zirconia buffer layers

In situ pulsed excimer laser processing of high Tc YBa2Cu3O7−δ films on Siu2009(100) substrates with yttria‐stabilized zirconia (YSZ) buffer layers, has been carried out for the first time using a multitarget deposition system. Both YSZ and YBa2Cu3O7−δ layers were deposited sequentially using a KrF excimer laser (λ=248 nm) at substrate temperature of 650u2009°C. The morphology and structure of the buffer layers and YBa2Cu3O7−δ films were determined using x‐ray diffraction and transmission electron microscopy techniques. The superconducting transition temperature Tc (onset) of 90 K and Tc0 (zero resistance) of 82 K were achieved for YBa2Cu3O7−δ thin films on Si with YSZ buffer layers. An interesting result of this study was that good quality, highly textured YBa2Cu3O7−δ films with the c axis perpendicular to the substrate could be grown on single crystal as well as polycrystalline textured YSZ layers deposited on silicon substrates.

Mechanism of formation of 60° and 90° misfit dislocations in semiconductor heterostructures

Abstract A proposal is made and experimental evidence provided for a mechanism of formation of 60° glissile ( a 2 )〈101〉[111] (mixed) and 90° ( a 2 )〈110〉[001] (edge) misfit dislocations in semiconductor heterostructures such as Ge/Si and GaAs/Si. With increasing thickness of the epilayer the nature of dislocations was found to change from being predominantly 60° to mostly 90°. Calculations of atomic structures of dislocations using Stillinger-Weber interatomic potentials were done to show that it is energetically favorable for a 60° core to transform into a 90° core. Thus the strained layer system relaxes its energy by minimizing short-range core distortions as well as long-range elastic displacements. A distinguishing feature of the present mechanism is that the second 60° dislocation is nucleated near the surface in the appropriate glide plane to meet (primarily via a glide process and partly by a climb process) and react with the first 60° dislocation which is already at the interface. High resolution transmission electron microscopy studies have been performed to rationalize and confirm the proposed mechanism.

Growth of ceramic thin films on Si(100) using an in situ laser deposition technique

We have investigated the formation of MgO and yttria‐stabilized ZrO2(YSZ) thin films on Si(100) substrates using laser (wavelength 248 nm pulse duration 40 ns, and repetition rate 5 Hz) physical vapor deposition method. The films were deposited from solid targets of MgO and polycrystalline YSZ in appropriate ambient with the substrate temperature optimized at 650u2009°C. The absorption coefficient in the MgO target was enhanced by Ni doping. The films were characterized using scanning and transmission electron microscopy (plan and cross section), x‐ray diffraction, and Rutherford‐backscattering spectrometry. The films were found to be polycrystalline with a texture. The thin films of MgO exhibited 〈111〉 texture, while the YSZ films contained both 〈111〉 and 〈200〉 textures.

In situ single‐chamber laser processing of YBa2Cu3O7−δ superconducting thin films on yttria‐stabilized zirconia buffered (100)GaAs

Superconducting YBa2Cu3O7−δ(YBCO) thin films have been deposited in situ on GaAs(100) by laser evaporation using yttria‐stabilized zirconia (YSZ) as a buffer layer. The YSZ buffer layer was deposited at room temperature initially, followed by a deposition at 650u2009°C. The YBCO layers were deposited subsequently at substrate temperature of 650u2009°C. All the depositions were carried out in a single chamber equipped with a multitarget holder using KrF excimer laser, λ=248 nm. The morphology and structure of the buffer layer and YBCO films were determined using x‐ray diffraction, scanning electron microscopy, transmission electron microscopy, and Auger spectroscopy. The nature of YSZ films was determined to be polycrystalline with partial texturing. The YBCO superconducting thin films on these YSZ‐coated GaAs specimens exhibited strong (00l) orientation with peak intensities similar to those observed on single‐crystal YSZ substrates. The superconducting transition temperature Tc (onset) of 92 K and Tc0(zero resis...

Misfit dislocations in superconducting thin films on SrTiO3{010}

We have studied the nature of strain-induced dislocations during epitaxial growth of YBa2Cu3-O7−δ films on SrTiO3{010} substrate. From the contrast analysis under different diffraction vectors, it was determined that the dislocations of a[110] and [011] Burgers vectors are formed in (110) and (011) planes respectively. At the points of intersection, they react (a[110]+a[011]→a[101]) to form a[101] segments in (010) planes. We have analyzed the formation of dislocations near the free surface above a critical thickness to minimize the total energy of the system. The dislocations glide into the film and the substrate. The importance of these results in the fabrication of thin film superconducting devices is emphasized.

Laser Atomic Cleaning And Initial Stages Of Epitaxial Growth Of Germanium Films On (100) Silicon During Laser Physical Vapor Deposition

Thin films of epitaxial germanium were deposited on (100) silicon substrates which were atomically cleaned using a pulsed excimer laser ( wavelength 308 nm, pulse duration 45 nanosecond, and energy density (0.75 - 1 J cm-2 ) in high vacuum ( 3.0 x 10-7 torr ). Thin films of germanium were then deposited in similar vacuum using the excimer laser at a higher energy density (3.5-6 J cm-2) to ablate germanium from a single crystal target. The substrate temperature during cleaning and deposition ranged from 300 to 500°C showing this to be a low temperature process. The films were analyzed using cross-section and plan-view transmission electron microscopy and high-resolution transmission electron microscopy. These films show the initial stages of epitaxial growth and Ge/Si interface with absence of an intermediate oxide layer. The significant reduction in substrate temperature for the formation of high quality epitaxial films opens up many new areas of applications requiring reduced thermal-budget processing.

In‐Situ Fabrication of YBa 2 Cu 3 O 7‐x Superconducting Thin Films Directly on Silicon Substrates with T c0 > 77K

We report in‐situ fabrication of c‐axis textured YBa 2 Cu 3 O 7‐x superconducting thin films with T co > 77K on unbuffered silicon substrates by the biased pulsed laser evaporation (PLE) technique in the temperature range of 550‐650°C. At substrate temperatures below 550°C, no c‐axis texturing of the superconducting film was observed. The YBa 2 Cu 3 O 7‐x superconducting films were fabricated by ablating a bulk YBa 2 Cu 3 O 7 target by a XeCl excimer laser (λ = 308 nm, τ = 45 × 10 ‐9 sec) in a chamber maintained at an oxygen pressure of 0.2 torr . The thickness of the films was varied from 0.3 to 0.5 nm depending on the number of laser pulses. Extensive diffusion was observed in thin films deposited at substrate temperatures above 550°C. However, microstructurally, with increase in the substrate temperature the films exhibited larger grain size and greater degree of c‐axis texturing (measured by the ratio of the (005) and (110) X‐ray diffraction peaks). This was found to give rise to better superconducting properties with T co exceeding 77 K for YBa 2 Cu 3 O 7‐x films deposited on Si substrates at 650°C.

In-Situ Processing of Epitaxial and Textured High Tc Superconducting Hoba2Cu3O7.X Thin Films By Pulsed Laser Evaporation Technique

Excellent quality epitaxial and textured superconducting HoBa 2 Cu 3 0 7 . x (Ho 123) thin films have been fabricated on lattice matched (100) KTaO 3 and (100) LaAlO 3 , and lattice mismatched (100) MgO substrates by the pulsed laser evaporation (PLE) technique. A bulk Hol23 target was evaporated using nanosecond excimer laser pulses with the evaporating material depositing on a substrate maintained in the temperature range of 550‐650°C. The temperature for zero resistance for HoBa2Cu 3 O 7 _ x films deposited on various substrates at 650°C varied between 85 to 89K. The epitaxial films deposited on (100) LaA10 3 substrates exhibited critical current densities greater than 3.5 x 10 6 Amps/cm 2 at 77 K. The superconducting properties of the Ho 123 films were found to be similar to Y123 films.