We have prepared (11n) oriented Bi2Sr2CaCu2O8+x (Bi2212) thin films by metal-organic decomposition method. The vicinal (110) substrate of SrTiO3 (STO) inclined toward the [1¯10] direction was used for suppressing “c-twinning”. In the sample prepared in the O2 atmosphere, only the (11n) peak appeared in the θ-2θ X-ray diffraction pattern. When the miscut angle of the substrate was φ = 10°, it was shown by the scanning electron microscope images and the (008) pole figures that the c-axis was inclined by about 38° and the c-twinning was substantially suppressed. ρab and ρc of this sample were calculated by the corrected van der Pauw method and component calculation of the two-dimensional resistivity tensor. The superconducting transition temperatures were Tc = 81 K. The temperature dependence of ρab was a typical metallic behavior reflecting the characteristics of the ab-plane of Bi2212. On the other hand, that of ρc did not become a typical semiconductor behavior in the c-axis direction of Bi2212, but it showed a metallic behavior. The anisotropic parameter γ was about 10.

Bi2Sr2CaCu2O8+x (Bi2212) has a layered perovskite structure and forms an intrinsic Josephson junction (IJJ). When a voltage is applied in the c-axis direction, an alternating current with a frequency proportional to the voltage is generated. A vibration mode excited by matching the frequency of the alternating current with a cavity resonance frequency can be used for a terahertz oscillator, which is considered to be a promising device that fills a frequency domain called the “terahertz gap”. It has already been proved that a μW class continuous coherent terahertz wave can be oscillated.1–4 The oscillation power is up to 0.61 mW when forming a three array structure with the IJJ stack as a unit.2 The oscillation frequency of this device can be tuned by bias current and hot bath temperature.3 Its frequency bandwidth is up to 1.9 THz (0.5 ∼ 2.4 THz).4 

The major preparation methods thus far have been reported on c-axis oriented Bi2212 which requires complicated procedures and dry etching, such as a precisely controlled etching process in depth direction in order to form the c-axis current paths. If a non-c-axis oriented thin film of which the c-axis parallel or incline to the substrate surface can be prepared, planar type IJJ devices can be fabricate simply by forming current paths parallel to the substrate.5,6 In preparing such non-c-axis oriented thin films, it is important that selection of substrates focusing on lattice matching. Furthermore, since lattice constants of the a-axis and the b-axis of Bi2212 are very close (a = 5.414 Å, b = 5.418 Å),7 usually so-called “ab-twinning” is formed.8 Since the ab-twinning is in-plane 90° misorientation defects,8 it is necessary to be suppressed. As a one of substrates satisfying these requirements, vicinal (100) substrates of SrTiO3 (STO) had been used, of which (100) plane of the substrate was inclined by 4 ∼ 6° toward the [111] direction.5,9–11 STO is a cubic crystal having a lattice constant a = 3.905 Å,12 and the length of the unit cell in the [110] direction is 5.522 Å. These lengths are close to those of the a-axis and the b-axis of Bi2212. In the b-axis direction the “incommensurate modulation” exists,13 which acts to minimize the interface energy between Bi2212 and the step edge of the substrate.14 Therefore, the Bi2212 crystal grows so that the b-axis is perpendicular to the step edges and the a-axis is along the edges. As a result, Bi2212 films of which c-axis is inclined by 4 ∼ 6° from the normal direction of the substrate are formed. The resistivity of the thin films in the step edge direction includes the b-axis and the c-axis component.9 Such orientation control of Bi2212 (and Bi2Sr2Ca2Cu3O10+x) was attempted on various other substrates. For example, STO(110) flat substrate,15,16 STO(110) vicinal substrate toward the [1¯10] direction,17–22 NdGaO3 substrate,18 MgO substrate,14 LaAlO3 substrate.23,24 However, all of these are vacuum processes such as ion beam sputtering,5,9,14 molecular beam epitaxy,6,11,23 atomic layer epitaxy,10 RF magnetron sputtering,15,16 DC magnetron sputtering,17 and metal organic chemical vapor deposition.18–22,24

If it is possible to prepare non-c-axis oriented thin films by a solution method, planar type IJJ devices can be fabricated by a simple process such as heat treatment after applying solution by a printing method. The metal-organic decomposition (MOD) is a kind of chemical solution deposition method which is an attractive fabrication technology in terms of cost and simplicity.25 It has the following features: Raw materials and apparatuses are inexpensive, it is possible to blend, prepared by a combination of most of raw materials, and can prepare large area thin films. In the case of preparing Bi2212 thin films, c-axis oriented thin films were formed by using STO(100) substrates,26,27 and (117) oriented thin film with c-axis twin structure (“c-twinning”) are formed by using STO(110) substrates.28 In order to fabricate Bi2212 IJJ devices, it is necessary to prepare thin films having no c-twinning in order to form c-axis current paths. Moreover, it is necessary to suppress ab-twinning. The c-axis lattice constant of Bi2212 is c = 30.6 ∼ 30.9 Å,7,13 which is relatively close to 8 times the lattice constant of STO (3.095 × 8 = 31.24 Å). If it is possible to grow so that the c-axis and b-axis of Bi2212 match the [001] direction and the [1¯10] direction of the STO(110) substrate respectively, as a result of suppressing “c-twinning” and “ab-twinning”, a-axis oriented thin films may be formed. However, as mentioned above, actually (117) oriented thin film is formed.28 According to Refs. 17 ∼ 22, it is possible to form (11n) oriented thin film without “c-twinning” by using STO(110) vicinal substrates toward the [1¯10] direction.

By the way, in Ref. 28, results of four-terminal resistance measurement by attaching electrodes to the four corners of the sample show that resistivities of these films have very large in-plane anisotropy, and its temperature dependence has a double transition behavior with resistivity drops and minimums. In order to explain these behaviors, it is necessary to separate the resistivity for each perpendicular direction from the measured four-terminal resistances, which are parallel to the substrate. The van der Pauw (vdP) method29–32 is effective to easily evaluate the resistivity of entire thin films. Typically, the vdP method is applied for isotropic materials. According to Refs. 31 and 32, it is also applicable for anisotropic materials by making appropriate correction. The resistivity of each direction of anisotropic materials can be evaluated by this method, which is called the “corrected vdP method”. It is a convenient alternative to the Hall-bar geometry.

In this study, focusing on suppression of c-twinning, we report fabrication of (11n) oriented Bi2212 thin films using vicinal STO(110) substrate by MOD method. Evaluation of the thin film was carried out through X-ray diffraction (XRD) pattern and pole figure, observation of surface morphology by scanning electron microscope (SEM) image, measurement of resistivity temperature dependency by corrected vdP method.

Bi2212 thin films under study were prepared by the MOD method using a stoichiometric BSCCO metal organic (MO) solution (supplied by Kojundo Chemical Lab. Co., Ltd. SK-BSCCO008). Substrates used were flat and vicinal STO (110) substrates with the size of 10 × 10 × 0.5 mm and the miscut angle φ = 10 and 20° toward the direction [1¯10]. A flat substrate of STO(100) was used to prepare c-axis oriented thin films for estimating the lattice constant of Bi2212. In the following, the flat substrates were expressed as φ = 0°. Preparation procedures were as follows:

  1. An amount of 6μ BSCCO MO solution was dropped onto substrates using a digital micropipette.

  2. The dropped solution was spin-coated by a 2-step process with 500 rpm for 5 sec and 3000 rpm for 1 min.

  3. The spin-coated films were dried at 120 °C for 40 min.

  4. The dried films were annealed at 810 °C for 30 ∼ 120 min in O2 or Air atmosphere.

  5. Furnace cooling were carried out.

The rotational speed of the spin-coating determines the film thickness d. According to Ref. 27, in the case of preparing the Bi2212 thin films, when the spin-coating is performed at 3000 rpm, the film thickness d becomes about 40 nm. The samples were taken into the box furnace at room temperature. Then, the samples were taken out after being cooled to room temperature. Figure 1(a) shows the temperature profile of a box furnace corresponding to the above steps (3) ∼ (5). When setting the O2 atmosphere, 100% O2 gas was kept flowing to the box furnace from the start of annealing. The preparation condition of the samples is shown in Table I. S-810-120 is a sample prepared to estimate the c-axis length of the Bi2212 thin films. The crystal structures of the samples were investigated by θ-2θ XRD patterns and (008) pole figures with CuKα radiation (λ = 1.54 Å). The surface morphologies were observed by SEM. Figure 1(b) and (c) shows a top view of an electrical measurement configuration and a typical SEM image of sample surface. x- and y-direction corresponded to a direction along and transverse to the grains, respectively. In the θ-2θ XRD pattern measurement, X-rays were irradiated from +x and –x directions, which was within the xz plane. Full width at half maximum (FWHM) and integral intensity were estimated from the θ-2θ XRD pattern. The electrodes were attached by silver paste. The resistance R12,43 was defined as V43/I12, where the voltage V43 and the current I12 were measured using contacts 4-3 and 1-2, respectively. R14,23 was defined in the same way.

FIG. 1.

(a) Temperature profile of a box furnace. The samples were taken into the box furnace at room temperature. Then, the samples were taken out after being cooled to room temperature. (b) Schematic drawing of a top view of the sample measured by corrected vdP method. The [001] direction of the STO substrate corresponds to the -x direction as indicated by the arrow. (c) SEM image of typical surface morphology of sample. (d) Schematic drawing of a side view of the sample. The angle formed by the [010] and [100] direction of the STO substrate and the substrate surface is expressed as 45° – φ and 45° + φ, respectively (φ is the miscut angle toward the direction [1¯10]).

FIG. 1.

(a) Temperature profile of a box furnace. The samples were taken into the box furnace at room temperature. Then, the samples were taken out after being cooled to room temperature. (b) Schematic drawing of a top view of the sample measured by corrected vdP method. The [001] direction of the STO substrate corresponds to the -x direction as indicated by the arrow. (c) SEM image of typical surface morphology of sample. (d) Schematic drawing of a side view of the sample. The angle formed by the [010] and [100] direction of the STO substrate and the substrate surface is expressed as 45° – φ and 45° + φ, respectively (φ is the miscut angle toward the direction [1¯10]).

Close modal
TABLE I.

Summary of sample labels and their annealing conditions.

Sample labelSubstrateMiscut angle φ (°)Annealing time ta (min)Atmosphere
S-810-120 STO(100) 120 O2 
S0-810-120-a STO(110) 120 Air 
S0-810-120 STO(110) 120 O2 
S10-810-120 STO(110) 10 120 O2 
S10-810-60 STO(110) 10 60 O2 
S10-810-30 STO(110) 10 30 O2 
S20-810-120 STO(110) 20 120 O2 
Sample labelSubstrateMiscut angle φ (°)Annealing time ta (min)Atmosphere
S-810-120 STO(100) 120 O2 
S0-810-120-a STO(110) 120 Air 
S0-810-120 STO(110) 120 O2 
S10-810-120 STO(110) 10 120 O2 
S10-810-60 STO(110) 10 60 O2 
S10-810-30 STO(110) 10 30 O2 
S20-810-120 STO(110) 20 120 O2 

A two-dimensional (2D) anisotropic properties were characterized by resistivity with principal resistivities ρx, ρy determined from measured four-terminal resistances R12,43 and R14,23 using corrected vdP method.31,32 The temperature at which the dρ/dT was maximum was defined as a superconducting critical temperature Tc. The ratio of the four-terminal resistance and the resistivity in each direction are defined as AvdP = R14,23/R12,43 and A = ρy/ρx, respectively.31 In the case of a square sample, the relationship between AvdP and A is as follows:32 

AvdPπ8ACexpπ2A, where C=2ln2π0.441.
(1)

Resistivity obtained by normal vdP method29,30 is defined as ρave. Relationship between ρave, ρx, ρy and four-terminal resistance is as follows:

ρave=ρxρy=πdln2R12,43+R14,232fAvdP,
(2)

where d is thickness of thin films and factor f is a function of AvdP, respectively. By using the resistivity ratio A and Eq. (2), ρx, ρy are determined as follows:

ρx=ρaveA,ρy=ρaveA.
(3)

The factor f satisfies the following relation:

AvdP1AvdP+1=fln2cosh112expln2f.
(4)

From above formulae (1) – (4), the procedure for obtaining ρx, ρy is as follows:

  • Resistivity ratio A is calculated by using Eq. (1). A is numerically calculated by Newton’s method.

  • The factor f is determined by using Eq. (4). This is also calculated by Newton’s method.

  • ρave is calculated by using Eq. (2).

  • ρx, ρy are calculated by using Eq. (3).

ρab and ρc can be obtained from ρx, ρy obtained by the above procedure. The procedure is as follows: ρx is the longitudinal direction of elongated plate-like crystal grains and the resistivity in this direction is considered to be dominant by ρab as follows.

ρab=ρx
(5)

As shown in Fig. 1(d), ρy is the resistivity in the direction crossing plate-like crystal grains parallel to the substrate, and it is considered that components ρab and ρc are included, which are perpendicular to each other. According to Refs. 6, 9, and 23, ρy is expressed by the following equation using the angle χ with ρab as a component of the anisotropic 2D resistivity tensor.33 

ρy=ρabcos2χ+ρcsin2χ
(6)

where, χ is the angle between the c-axis direction of crystal grains and the normal direction of the substrate as shown in Fig. 1(d). By the peak value of the tilting angle α of the horizontal in the pole figure of the (008) plane, it can be expressed by the following equation.

χ=90°α
(7)

The θ-2θ XRD pattern of sample S-810-120 (not shown in figure) was a typical pattern of c-axis oriented thin film in which only the 00 diffraction peaks (and the substrate peak) appeared. The 006, 008, 0010, 0012, and 0020 diffraction peaks were 2θ = 17.31°, 23.12°, 29.01°, 34.96° and 60.01°, respectively. From these values, c = 30.82 Å was obtained by extrapolation method using Nelson-Riley function. Figure 2 shows comparison of XRD patterns of samples with different atmospheres ((a)Air and (b)O2) and miscut angles ((b)0°, (c)10° and (d)20°) using STO(110) substrates. Only peaks of Bi2212 phase and STO substrates appeared in these XRD patterns. Comparing the XRD patterns of the samples with φ = 0° in Fig. 2 (a) and (b), the 00 peaks were very small when annealing was performed in the O2 atmosphere. This suggests that growth of c-axis oriented crystal grains was suppressed. This tendency was similar in the case of using vicinal substrates with φ = 10°. Comparing the XRD patterns of the samples annealed in the O2 atmosphere in Fig. 2(b) and (c), the 119 and 115 peaks of similar intensity appeared in the case of φ = 10°, whereas the large 117 peak appeared in the case of φ = 0°. In the case of using vicinal substrates with φ = 20°, the 11n peaks hardly appeared of the samples prepared in either O2 or Air atmosphere. Therefore, it is considered that this substrate is not suitable for preparation of non-c-axis oriented thin films.

FIG. 2.

θ-2θ XRD patterns of samples with different atmospheres ((a) Air and (b) O2) and miscut angles ((b) 0°, (c) 10° and (d) 20°) using STO(110) substrates. (e) and (f) are FWHM and integrated intensity with different annealing time ta = 30 ∼ 120 min for a fixed miscut angles φ = 10°.

FIG. 2.

θ-2θ XRD patterns of samples with different atmospheres ((a) Air and (b) O2) and miscut angles ((b) 0°, (c) 10° and (d) 20°) using STO(110) substrates. (e) and (f) are FWHM and integrated intensity with different annealing time ta = 30 ∼ 120 min for a fixed miscut angles φ = 10°.

Close modal

Figures 2(e) and 2 (f) show the annealing time dependence of FWHM and integral intensity of the 115, 119 and 0010 peaks of samples annealed in O2 atmosphere (S10-810-120, S10-810-60 and S10-810-30) using vicinal substrates with φ = 10°, respectively. Each marker represents the average of the measured values in the +x and -x direction, which is showed in Fig. 1, and each error bar represents the variation. The FWHM of the 119 and 115 peaks hardly changed in the range of ta = 30 ∼ 120 min, whereas the FWHM of the 0010 peak tended to become smaller with the annealing time. The integrated intensity of the 119 peak was dominant in the range of ta = 30 ∼ 60 min, whereas the integrated intensity of the 115 and 0010 peak tended to become larger with the annealing time. These results suggest that growth of (119) oriented crystal grains is sufficiently dominant in the range of ta = ∼ 60 min, whereas (115) and c-axis orientated grains growth progresses with longer annealing time. Therefore, it is considered appropriate to set the annealing time to about 60 min, because it may be desirable to have a single orientation when used as IJJ devices.

Figure 3 shows a comparison of the observed surface morphologies by SEM and (008) pole figures with φ = 0 and 10° for ta = 60 min. In-plane direction β in the pole figure corresponds to that of the SEM image. The β value in pole figures corresponding to the direction of the white arrow in the SEM image is β = 180°, and this direction corresponds to the STO[001] direction. As shown in Figs. 3(a) and 3(b), elongated plate-like crystal grains were observed in the SEM images of both samples. The lengths of the crystal grains along Bi2212 [1¯10] direction were about several μm. As shown in Fig. 3(c), in the case of φ = 0°, the (008) poles clearly appeared similarly at β = 90° and 270°. On the other hand, as shown in Fig. 3(d), in the case of φ = 10°, clear (008) pole appeared at β = 270° but hardly appeared at β = 90°. Figure 3(e) and (f) are plots of the reflected X-ray intensity vs α characteristics (I - α characteristics) for β = 90° and 270°, respectively. In the case of φ = 0° in Fig. 3(e), the average of the peak positions at β = 90° and 270° was α = 41° (χ = 49°). This suggests that c-twinning are formed in which the c-axis directions are inclined by 49° toward both β = 90° and 270° from the normal direction of the substrate. In addition, this angle of the peak position coincided with the inclined angle of the c-axis of the (117) oriented crystal grain. However, the peak FWHM was about 9°, which was very broad.

FIG. 3.

The observed surface morphologies by SEM ((a) φ = 0° and (b) 10°) and (008) pole figures ((c) φ = 0° and (d) 10°) for ta = 60 min. (e) and (f) are plots of the reflected X-ray intensity vs α (I - α) characteristics for β = 90° and 270° ((e) φ = 0° and (f) 10°).

FIG. 3.

The observed surface morphologies by SEM ((a) φ = 0° and (b) 10°) and (008) pole figures ((c) φ = 0° and (d) 10°) for ta = 60 min. (e) and (f) are plots of the reflected X-ray intensity vs α (I - α) characteristics for β = 90° and 270° ((e) φ = 0° and (f) 10°).

Close modal

In the case of φ = 10° in Fig. 3(f), a large peak appeared at β = 270°, and its peak position was α = 52° (χ = 38°). This was relatively close to the inclined angle of the c-axis of (1110) oriented crystal grains of Bi2212 (= 38.8°). Since the FWHM of the peak was very broad (about 7°), this is consistent with the fact that the large 119 peak appears on the θ-2θ XRD pattern in Fig. 2(c). According to the ICDD data of Bi2212,34 the 1110 peak does not appear. On the other side β = 90°, very broad low peaks appeared at α = 32° (χ = 58°) and α = 47° (χ = 43°). They were close to the inclined angle of the c-axis of (115) oriented grains (inclined angle 58.1°) and (119) oriented grains (inclined angle 41.8°), respectively. The relative intensities of 115 and 119 peak are 95 and 16, respectively.34 Therefore, for the crystal grains of these orientations at β = 90°, it is considered that the component of (115) oriented grains appeared as a large peak in the θ-2θ XRD pattern of Fig. 2(c), and in contrast, the one of (119) contributed little. Therefore, it is suggested that the c-axes of most crystal grains are inclined by about 38° toward only one direction of β = 270°, and the formation of c-twinning is suppressed.

Figure 4 show schematic views (imaginary views) of lattice matching. The lengths of the a-axis and the b-axis of Bi2212 were set to the same average length (= 5.416 Å) in these figures. Figure 4(a) is a top view of lattice matching in the case of using a flat substrate of STO(100). Figure 4(b) ∼ (d) show lattice matching of the sample using the STO(110) substrate with the miscut angle φ = 0° as seen from three directions. In each figure, Bi2212 unit cell in the case of α = 45° is shown. Figure 4(d) shows the angle in the range of FWHM of the I - α characteristic in Fig. 3(e) (α = 36 ∼ 45°). Figure 4(e) shows lattice matching seen from the STO[001] direction of the sample using the STO(110) substrate with the miscut angle φ = 10°. In the figure, the angle in the range of FWHM of the I - α characteristic in Fig. 3(f) (α = 47 ∼ 54°) and the Bi2212 unit cell in the case of α = 54° are shown.

FIG. 4.

Schematic views (imaginary views) of lattice matching for Bi2212 and STO substrates. (a) is a top view in the case of using a flat substrate of STO(100). (b) ∼ (d) show lattice matching of the sample using the STO(110) substrate with the miscut angle φ = 0° from three directions ((b) view from STO[110], (c) STO[100] and (d) STO[001]). (e) shows lattice matching of the sample using the STO(110) substrate with the miscut angle φ = 10° from STO[001].

FIG. 4.

Schematic views (imaginary views) of lattice matching for Bi2212 and STO substrates. (a) is a top view in the case of using a flat substrate of STO(100). (b) ∼ (d) show lattice matching of the sample using the STO(110) substrate with the miscut angle φ = 0° from three directions ((b) view from STO[110], (c) STO[100] and (d) STO[001]). (e) shows lattice matching of the sample using the STO(110) substrate with the miscut angle φ = 10° from STO[001].

Close modal

As described above, c-twinning was suppressed in the sample with miscut angle φ = 10°. Fig. 5(a) shows the temperature dependence of ρab and ρc of the sample S10-810-60. These values were calculated according to equations (5) – (7) using the c-axis inclined angle χ = 38° at the peak position of β = 270° in Fig. 3(f). As described in the experimental section, the film thicknesses are assumed to be d = 40 nm.27 In both ρab and ρc, the critical temperature was Tc = 81 K, indicating metallic behavior. Fig. 5(b) shows the annealing time dependence of Tc and anisotropy parameter γ (= (ρc/ρab)1/2 (at Tc)) with miscut angle φ = 10° in O2 atmosphere. γ became smaller as the annealing time ta became longer. ρc shows semiconductor behavior when γ is about 100 or more.6,9 Even when used as an IJJ device, γ value greater than that is necessary, and for this purpose it is necessary to reduce the oxygen content in Bi2212.5,6,35 However, from the results of XRD, annealing in O2 atmosphere is required for non-c-axis oriented thin film preparation. In order to reduce the oxygen content, it is considered effective to perform Ar annealing35 after thin films prepared.

FIG. 5.

(a) ρ-T curves for annealing time ta = 60 min and miscut angles φ = 10° in O2 atmosphere. (b) Annealing time ta dependencies of critical temperature Tc and anisotropy parameter γ with miscut angle φ = 10° in O2 atmosphere.

FIG. 5.

(a) ρ-T curves for annealing time ta = 60 min and miscut angles φ = 10° in O2 atmosphere. (b) Annealing time ta dependencies of critical temperature Tc and anisotropy parameter γ with miscut angle φ = 10° in O2 atmosphere.

Close modal

In order to realize fabrication of non-c-axis oriented thin film that can be used for Bi2212 IJJ device by non-vacuum process, we attempted to fabricate non-c-axis oriented thin film without c-twinning by MOD method. For that purpose, focusing on lattice matching, vicinal STO(110) substrates inclined toward the [1¯10] direction was used. The crystallinity and orientation were evaluated by the θ-2θ pattern and the (008) pole figure of XRD. The temperature dependence of ρab and ρc was evaluated using corrected vdP method and resistivity tensor. As a result, the followings were clarified.

  1. In comparison between Air and O2 atmosphere, the O2 atmosphere is more suitable for growth of crystal grains with (11n) orientation.

  2. In the case of using a substrate with a miscut angle φ = 10° inclined toward the [1¯10] direction, it was revealed that the (119) oriented Bi2212 thin film prepared had almost no c-twinning.

  3. Growth of c-axis oriented Bi2212 grains tended to be suppressed by decreasing the annealing time.

  4. ρab showed a typical metallic temperature dependence. This suggests that it well reflected the resistivity of Bi2212 in the ab-plane direction. ρc was not expected semiconductor characteristics and the anisotropy parameter γ was also very small.

In this report, we succeeded in almost suppressing c-twinning, but the large spread of orientation axes may be an obstacle to the fabrication of Bi2212 IJJ device, which is a problem to be solved in the future. In addition, suppression of ab-twinning, which is another obstacle for IJJ device fabrication, will be discussed elsewhere.

The pole figure measurements by XRD were performed at the Nagaoka University of Technology Analysis and Instrumentation Center. We thank Nagaoka University of Technology Analysis and Instrumentation Center for use of facilities and equipment.

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