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2.4 Preparation of Superconducting Thin Films

Deposition of superconducting thin films can be accomplished by a variety of methods as seen in Table 2.2. Each method has certain advantages over the others which yield various degrees of thin film quality. There are six major classes of deposition techniques: thermal evaporation, sputtering, chemical vapor deposition, metalorganic solution deposition, electrodeposition of precursors, and pulsed laser deposition.

Thermal evaporation23 from a single source is one of the most simple routes to deposition. While this method is effective in the deposition of single element superconductors like Pb, it is a poor choice in the deposition of the more complex cuprate superconductors. These films are often deficient in one or more elements. On the other hand, with molecular beam epitaxy, layer by layer growth can be achieved with independently controlled multiple sources. This method allows researchers to build a superconducting crystal structure that may not be realized by bulk processing. The sources are locally heated by an electron beam directed toward the sources.

Deposition by sputtering23 can be accomplished by either on-axis or off-axis deposition with Ar (or other inert gases) plasma and a small amount of O2. On-axis sputtering positions the substrate directly in front of the target. The deposition rates can be as rapid as 0.1 nm/s. The deposited film gets directly exposed to the plasma causing resputtering of some elements in the thin film. This treatment alters the stoichiometric composition of the material. For this reason, off-axis sputtering is preferred. Here, the substrate is oriented at a right angle from the target. This sidesteps the resputtering problem but results in slow deposition rates of 0.3 /sec. In magnetron sputtering, the plasma created by either an rf or dc potential is directed with a permanent magnet.

Chemical vapor deposition23 has the advantage of fast deposition rates and uniform coating of large substrates. This method employs volatile metalorganic precursors as sources for the metallic elements in the superconductor’s composition. There are three qualities required of the precursor materials: high vapor pressure for vapor transport, stability at their operating temperature, and stability at room temperature to prevent rapid degradation prior to use. The precursors are heated during transport to ~250oC with an Ar carrier gas. They are then combined where they are deposited onto a substrate heated at 800oC-900oC, depending on the material. The two obstacles that need to be overcome are the identification of proper volatile metalorganic reagents and the contamination of the final film from the organic portion of the precursor material.

Solution deposition using metalorganic23 compounds has been successfully used as an ex situ method. A solution of the metalorganic compounds is spin-coated onto a substrate. The substrate is then heated to form the high-temperature superconducting film. Use of inert substrates or non-reactive buffer layers is critical for this technique because of the higher temperatures needed to form the thin films. This method is also plagued by contamination of residual carbon from the precursors as well as by substrate interdiffusion problems.

Electrodeposition of precursor films18 has been used to deposit a wide variety of high-temperature superconductor materials. In this method, metal hydroxides are deposited on the surface of an electrode. The material is subsequently heated to form the superconducting material. One difficulty in forming stoichiometric amounts of each metal is that often the deposition potential of each metal is different. This issue has been overcome by empirically changing the concentration of each metal in solution as well as by varying the deposition potentials. One problematic occurrence in forming YBa2Cu3O7-x thin films is the production of BaCaO3 from Ba(OH)2. The stability of BaCaO3 hinders the formation of these thin films because the decomposition temperature of BaCaO3 is relatively close to that of the melting point of YBa2Cu3O7-x. Unfortunately, YBa2Cu3O7-x melts incongruently (i.e. it decomposes upon transformation to the liquid phase). While this technique shows promise for large-scale applications, the quality of the films has not yet achieve the same quality as other more refined techniques previously mentioned.

Pulsed laser deposition of high-temperature superconducting thin films has been one of the most successful deposition techniques employed. In this method, a UV laser, typically a 248 nm KrF excimer laser, is directed at a dense, rotating polycrystalline ceramic pellet of YBa2Cu3O7-x in a partial atmosphere of O2.

Deposition method

Substrate

Tc(zero) (K)

Jc (A/cm2), 77 K

Ref

off-axis sputtering

LaAlO3

MgO

92

85

4.8x106

6x107 (4.2 K)

24

25

molecular beam epitaxy

MgO

MgO

82

89

1x105

1x106

26

27

electron beam epitaxy

ZrO2

SrTiO3

MgO

86

90

87

2x105

4x106

1x106

28

28

28

pulsed laser deposition

SrTiO3

SrTiO3

YSZ

90

92

89

5x106

3x106

1x106

29

29

29

electrodeposition

ZrO2

91

360

30

MOCVD

YSZ

SrTiO3

84

92

5x105

2x106

31

32

Table 2.2: Comparisons of transport properties of YBa2Cu3O7 thin films generated from various deposition techniques. (Adapted from reference33)

The laser locally heats the sample to produce a plume of atoms, molecules, and radical species which is directed at a heated substrate. Thin films made with this method can be deposited at higher rates than that of off-axis sputtered films with a deposition rate of ~10 /sec or faster.

2.4.1 Substrate Requirements

The choice of substrates is critical for making high quality, epitaxial, superconducting thin films. There are five basic requirements33 that need to be considered when choosing a substrate. They are: good lattice match, non-reactive substrates, compatible thermal expansion with the superconductor, smooth surfaces, and suitability for device fabrication.

Good lattice match at the deposition temperature, as well as similarities in the crystal structure of substrates and thin films, favors epitaxial film growth. The properties of some common substrates used for deposition of YBa2Cu3O7-x are shown in Table 2.3. Of these substrates, SrTiO3, LaAlO3, NdGaO3, and LaGaO3 have perovskite crystal structures which are closely related to the crystal structure of YBa2Cu3O7-x. Their small lattice mismatch and perovskite structure make them a good choice for substrates. The compound, MgO, is another commonly used substrate that has a relatively small lattice mismatch upon a 45o rotation. Although the lattice mismatch for MgO is slightly higher and it has a primitive cubic rather than cubic perovskite crystal structure, it is widely used because of its high melting temperature and its low interdiffusion with YBa2Cu3O7-x as will be discussed later.

Substrate

Structure

% Lattice mismatch

a b c/3

Dielectric constant

mp (oC)

SrTiO3

cubic (perov)

2.14 0.49 0.29

277

2030

MgO

cubic (NaCl)

45o rotation

10.20 8.41 8.19

3.90 2.21 2.00

9.7

2852

LaAlO3

rhomb (perov)

-0.81 -2.42 -2.62

23

2110

LaGaO3

ortho (perov)

1.80 0.15 0.67

25

1715

NdGaO3

ortho (perov)

1.07 -0.60 -0.77

20

1600

Al2O3

hexagonal

9.4 7.0 11.2

9.3

2050

Si

cubic

0.4 -2.4 -1.5

11.7

1410

YSZ

cubic

-3.6 -6.3 -5.8

25

2650

Table 2.3 Lattice mismatch and properties of common substrate materials for YBa2Cu3O7-x thin films.33, 34

Substrates that are non-reactive at the high deposition temperatures are essential in the formation of high-temperature superconducting thin films. The deposition temperature at the substrate is typically between 500-850oC. It is critical to have an inert substrate to avoid interdiffusion between the superconductor and substrate. There are three types of interactions between the substrate and superconductor. The first is the substitution of the substrate atoms into the lattice of the superconductor. Additionally, there could be formation of a secondary phase which acts as a passivating layer, preventing diffusion into the lattice. Finally, there can be formation of a secondary phase that does not passivate and allows more interdiffusion between the superconductor and the substrate. On substrates which are known to be reactive, the deposition of a thin buffer layer over the substrate before the growth of YBa2Cu3O7-x can inhibit the degradation of the film caused by substrate interdiffusion. Since materials synthesis and deposition conditions are the most advanced for silicon technology, it would be desirable to use silicon as a substrate material. The facile interdiffusion of silicon into YBa2Cu3O7-x at elevated temperatures causes severe chemical incompatibility between silicon and YBa2Cu3O7-x, preventing the direct deposition of the cuprate material onto single-crystal silicon. To avoid this problem, YSZ (yttria-stabilized ZrO22 buffer layers have been deposited onto the silicon substrates. Again, MgO exhibits the best thermal stability of the commonly used substrates, having the highest melting temperature as seen in Table 2.3. The thermal stability of MgO is demonstrated in the low interdiffusion at the interface of the substrate and the YBa2Cu3O7-x thin film.

Thermal expansion of the substrate and superconductor should be comparable for proper thin film deposition. Since the superconducting thin films must be deposited at temperatures of 500-850oC, poorly matched thermal expansion coefficients may result in severe cracking of the thin film upon cooling. Additionally, at low temperatures the difference in the thermal expansion coefficients will induce strain in the lattice from lattice mismatch. The extent of the consequent strain depends on the difference between the lattice strain at the deposition temperature and the strain at room temperature or lower operating temperatures. This strain is typically relaxed both through the formation of misfit dislocations and the occurrence of twinning structures. If the strain is not relaxed, cracking will result within the superconducting thin film.

The substrate quality strongly affects the deposition of superconducting thin films. Substrate defects such as crystal twinning, impurity phases, and structural inhomogeneity can be manifested in poor epitaxial growth of the thin film. Substrate roughness plays a role in the resulting superconductor film morphology and the formation of grain boundaries. When MgO substrates are annealed at 1100-1200oC, they exhibit well-defined surface steps, unlike non-annealed surfaces that possess few steps. The annealing process promotes surface regrowth and the creation of flat terraces on the substrate surface. Thin films of YBa2Cu3O7 deposited on these annealed MgO substrates show better electrical transport properties and less misorientated grains than those on chemically or mechanically polished substrates. Another related factor is choosing a substrate that does not have a phase transition over the wide temperature range needed for deposition (500-850oC) and operation (93 K or lower).

Fabrication of devices for some fundamental studies and commercial applications can place restrictions on some substrate materials. The substrates must withstand processing conditions used to fabricate the devices as well as be compatible with the operation of the device. For example, SrTiO3 meets the above specified four requirements, but it has a high dielectric constant, making it incompatible for microwave device applications. Although the substrate materials listed in Table 2.3 would be suitable, the fabrication of high-temperature superconducting electrodes on metallic substrates such as Ag would be inappropriate in many cases, because the electrochemical response of superconducting thin films would be mingled with that of the underlying substrate. The metallic substrate may even be oxidized at certain potentials leading to chemical damage of the electrode. Choosing an insulating substrate material would be a necessity under such conditions.

Although high quality superconducting thin films have been grown on an assortment of substrates, no single substrate can fulfill all of these requirements. The development of new substrate materials and novel processing conditions will likely yield a broader choice in substrates for YBa2Cu3O7-x and other high-temperature superconductors.

2.4.2 Thin Film Preparation By Pulsed Laser Ablation

The thin films used in the McDevitt laboratory are typically a-axis and c-axis oriented YBa2Cu3O7-x thin films prepared by pulsed laser ablation. A-axis thin films were deposited on polished LaAlO3 at a lower temperature35 of ~630oC than that of the c-axis thin films, which were deposited on polished MgO at ~750oC. Substrate temperatures and choice of substrates are two of the largest factors in determining the crystallographic orientation of the deposited thin films. A list of typical parameters normally used to prepare YBa2Cu3O7-x thin films is given in Table 2.4, and a schematic diagram of laser ablation can be seen in Illustration 2.3.

Laser source

Questek 2000

Wavelength

248 nm (KrF)

Energy of laser

350 mJ

Spot size of laser target

2x4 mm2

Energy density

1.5 J/cm2

Repetition rate

10 Hz

Pulse length

30 ns

Average deposition rate

1 /pulse

Base vacuum pressure

5x10-5 Torr

Oxygen partial pressure

100-150 mTorr

Target-to-substrate distance

5 cm

Substrate materials

MgO, LaAlO3, SrTiO3, Si with YSZ buffer

Substrate temperatures

630-850oC

Table 2.4: General pulsed laser deposition parameters for YBa2Cu3O7-x thin films.

Schematic drawing of the pulsed laser ablation / deposition system used to make thin films.


Illustration 2.3: Schematic drawing of the pulsed laser ablation / deposition system used to make thin films. (Adapted from reference36).
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Updated on: April 15, 2010 8:26 PM