YBa₂Cu₃O_7-δ single crystals revisited: Scanning probe data on very pure samples grown in BaZrO₃ crucibles

Article

Reference

YBa

Cu

O

single crystals revisited: Scanning probe data on very pure samples grown in BaZrO

crucibles

HUBLER, Ulrich, et al.

Abstract

Cu3O7-δ (YBCO) single crystals grown in BaZrO3 crucibles by scanning and friction force microscopy (SFM, FFM) as well as by scanning tunneling microscopy (STM) and angle resolved X-ray photo-electron spectroscopy (ARXPS). By STM and SFM, clean stepped terraces are observed on as-grown crystals. Friction contrast, however, implies the presence of different materials, presumably traces of flux. After oxidation of the YBCO crystals, particles of 10 to 100 nm in size appear on the surface. Their number per unit area increases with time when the crystal is stored in air. Complementary information on such aged YBCO surfaces is provided by ARXPS. These data reveal 3 nm of mainly BaCO3 and CuO on top of the YBCO matrix and suggest a very slow aging rate of the crystals, which is 20 times slower than for epitaxial thin films. The superconducting energy gap has been determined on fresh crystals by scanning tunneling spectroscopy (STS) at 4.6 K as 2Δ≈30 meV. By spatially resolved tunneling spectroscopy the vortex lattice in a magnetic field of 4 T has been imaged. The inter-vortex distance amounts to 20±2 nm.

HUBLER, Ulrich, et al . YBa

Cu

O

single crystals revisited: Scanning probe data on very pure samples grown in BaZrO

crucibles. Applied Physics A: Materials Science and Processing , 1998, vol. 66, no. Supplement 1, p. S1219-S1222

DOI : 10.1007/PL00022824

Available at:

http://archive-ouverte.unige.ch/unige:112216

Disclaimer: layout of this document may differ from the published version.

1 / 1

Applied Physics A

Materials

Science & Processing

Springer-Verlag 1998

YBa 2 Cu 3 O 7 −δ single crystals revisited: Scanning probe data on very pure samples grown in BaZrO ₃ crucibles

U. Hubler¹, P. Jess¹, H.P. Lang¹, A. Erb², E. Walker², M. Regier³, D. Schild³, J. Halbritter³, H.-J. Güntherodt¹

1Institute of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland

2DPMC, Universit´e de Genève, 24, quai Ernest Ansermet, CH-1211 Genève 4, Switzerland

3Forschungszentrum Karlsruhe, IMF 1 and INE, Postfach 3640, D-76021 Karlsruhe, Germany Received: 25 July 1997/Accepted: 1 October 1997

Abstract. We have studied very pure YBa2Cu3O_7−δ(YBCO) single crystals grown in BaZrO3 crucibles by scanning and friction force microscopy (SFM, FFM) as well as by scan- ning tunneling microscopy (STM) and angle resolved X-ray photo-electron spectroscopy (ARXPS). By STM and SFM, clean stepped terraces are observed on as-grown crystals.

Friction contrast, however, implies the presence of different materials, presumably traces of flux. After oxidation of the YBCO crystals, particles of 10 to 100 nm in size appear on the surface. Their number per unit area increases with time when the crystal is stored in air. Complementary informa- tion on such aged YBCO surfaces is provided by ARXPS.

These data reveal 3 nm of mainly BaCO3 and CuO on top of the YBCO matrix and suggest a very slow aging rate of the crystals, which is 20 times slower than for epitaxial thin films. The superconducting energy gap has been determined on fresh crystals by scanning tunneling spectroscopy (STS) at 4.6 K as 2∆≈30 meV. By spatially resolved tunneling spec- troscopy the vortex lattice in a magnetic field of 4 T has been imaged. The inter-vortex distance amounts to 20±2 nm.

Scanning probe methods and angle resolved X-ray photo- electron spectroscopy (ARXPS) are well established tools for studying the surface properties of high-temperature su- perconductors [1–4]. Each of these methods probes specific qualities and depths of a material, allowing the reconstruction of a three-dimensionally resolved interface. Scanning tunnel- ing microscopy (STM) and scanning force microscopy (SFM) not only allow investigations of the surface morphology on a nanometer to micrometer scale, but also provide material contrast by use of friction force microscopy (FFM). This yields valuable information on the purity of such surfaces.

Complementary data on their chemical composition and ag- ing process is provided by ARXPS. Furthermore, scanning tunneling spectroscopy (STS) probes the electronic density of states (DOS) near the Fermi level and allows the determin- ation of the superconducting energy gap from current vs. volt- age(I(V))characteristics [5].

Until recently, traces of residual flux on YBa2Cu3O_7−δ (YBCO) surfaces (mainly BaCuO2 and CuO serving as flux solvents) as well as impurities originating from crucible ma- terials (e.g. Al2O3, Au, MgO) constituted a major problem for crystal growers. Poor surface quality presents a problem es- pecially when a surface-sensitive technique, such as STM or STS, is applied to probe electronic properties in the supercon- ducting state. The short coherence length in YBCO, which is only of the order of a few Å in the [001] direction, means measurements are strongly affected even by thin contamina- tion layers.

These problems seem to have been overcome by using BaZrO3 as the crucible material [6–8]. This material shows a sharp and non-reactive interface with the flux used in the YBCO growth process. Furthermore, Zr is unlikely to be in- corporated into YBCO by substitution reactions, since its ion size does not fit in the structure of YBCO. Thus, impurities caused by the crucible material can be avoided in the flux as well as inside the single crystals or on their surface.

The purity of such crystals is shown by the fact that not only could the superconducting energy gap be determined by STS at 4.2 K, but also the vortex lattice on YBCO in a mag- netic field of 6 T could be imaged by STM for the first time by using such samples [9, 10].

We will present analysis of the YBCO(001) surface morphology of as-grown and fully oxygenated single crys- tals using various scanning probe microscopy methods (SFM, FFM and STM). The slow aging process of oxygenated crys- tals – even when stored under ambient conditions – will be illustrated by SFM and ARXPS. STM and STS at low tem- perature (4.6 K) have been used to probe the superconducting energy gap and to image the vortex lattice in a magnetic field of 4 T.

1 Experimental

The surface morphology of the (001) face of as-grown and oxygenated YBCO single crystals grown in BaZrO3 cru-

S1220

cibles has been studied by STM, SFM and FFM¹. No further treatment was applied to the YBCO surface. The experi- ments were performed in air at 300 K and ambient pressure.

One of the crystals was re-investigated by SFM and ARXPS after having been stored under ambient conditions for several months.

After scanning probe microscopy on the oxygenated single crystal had been performed, it was investigated by ARXPS. The spectra were obtained by a Physical Electron- ics PHI 5600ci small spot XP spectrometer (analyzed area

≈π(200µm)²)². The UHV base pressure during spectra ac- quisition was ≈1×10⁻⁹mbar. The crystal was fixed with silver paint on a copper sheet and placed on a modified stan- dard sample holder (axis of rotation parallel to a/b plane).

Beside wide scans, angle resolved spectra of Y 3d, Ba 3d, Ba 4d, Cu 2p, O 1s, C 1s and the valence band region were acquired. After the first ARXPS investigation the single crys- tal was exposed to ambient air for about 4 months and then re-investigated with the same experimental setup.

STS at 4.6 K was performed by using a home-built low- temperature STM operated in high vacuum. Cooling to liquid helium temperature was achieved with a helium exchange gas pressure of 1 mbar. The actual experiments were performed at 3.5×10⁻³mbar for high temperature stability reasons. I(V) characteristics were acquired by ramping the bias voltage with a deactivated feedback loop. The numerical derivative of these data yields the differential conductance d I/dV which is proportional to the electronic density of states (DOS) near the Fermi level and allows the determination of the super- conducting energy gap. In a magnetic field of 4 T along the [001] direction of the YBCO crystal d I/dV characteristics were acquired with a high lateral resolution. This allowed the spatial variation of the DOS in a magnetic field to be stud- ied. For spectroscopic measurements, the bias voltage was typically lowered to Ubias≈100 mV, whereas the tunneling current was raised to I_t≈400 pA to increase the signal-to- noise ratio. During acquisition of I(V) curves, the voltage was typically ramped from−50 to+50 mV.

2 Results and discussion

2.1 Surface analysis by scanning probe microscopy

The topography of YBCO single crystals grown in BaZrO3

crucibles as measured by SFM is shown in Fig. 1a. Unit cell steps in the [001] direction of YBCO (height≈1.2 nm) along

1STM experiments were carried out using mechanically sharpened Pt90Ir10

tips. Microfabricated Si cantilevers with an integrated tip and spring con- stants of 0.28–0.36 N/m were used for SFM and FFM measurements.

Typical imaging conditions were loading forces of F≈1 nN for SFM, tip bias voltages of Ubias≈1 V, and tunneling currents of It≈100 pA for STM.

2Both Mg K_α X-rays (Mg) and monochromated Al K_α X-rays (Al) were used, together with a concentric hemispherical electron energy analyser op- erated at constant pass energy of 23.5 eV (Mg) and of 5.85 eV (Al) leading to a FWHM of the metallic Ag 3d 5/2 line of 0.96 eV (Mg) and of 0.54 eV (Al) respectively. The spectrometer was calibrated against the Au 4f 7/2 line at 84.00 eV, the Ag 3d 5/2 line at 368.29 eV and the Cu 2p 3/2 line at 932.67 eV of metallic foils sputtered in UHV. The high resolution measure- ments (Al) were used to derive the exact binding energy position of lines, whereas the Mg-anode served for the angle resolved investigations because of its real “flat angle characteristic” (the intensity of a homogeneous sample is nearly constant over a wide range of electron take-off angles).

with smooth and clean terraces (labeled ‘T’) characterize this surface. Some islands ‘I’ with a height of ≈1 nm are ob- served close to step edges. FFM forward and backward scans exhibit a friction contrast ofµI/µT≈1.5 between islands and terraces [11]. This implies that the islands are of a different material, probably minor traces of the flux used in the growth process of YBCO. Additionally the terraces are divided in two regions parallel to step edges: the region marked ‘L’ is

≈0.3 nm lower than the one labeled ‘H’. This might be due to different termination layers. STM measurements yield simi- lar results [11]. The islands, however, appear about 4 Å lower in STM images than the terraces, which may possibly be at- tributed to differences in the work function and a reduction of the tunneling probability because of single-electron tunneling effects [12]. So far, we have not been able to obtain atomic resolution on these surfaces.

Other crystals exhibit ribbon-like (labeled ‘R’ in Fig. 1b), star-like or checkerboard-like structures (labeled ‘S’ and ‘C’, respectively, in Fig. 2a, which shows an oxygenated crystal) on the terraces in SFM images. Ribbon-like features show no friction contrast and are therefore believed to be growth structures, possibly related to twin boundaries of YBCO. On star-like or checkerboard-like structures the friction coeffi- cient is lower by a factor of≈0.6 compared to the underlying YBCO matrix material.

After the crystals have been oxygenated to establish su- perconductivity, additional particles ‘P’ appear on the sur-

1 µm

(a)

H L T

I

Fig. 1.

(b)

1 µ m R

a SFM topography image of a YBCO single crystal grown in a BaZrO3crucible. Clean terraces ‘T’ divided in two regions ‘H’ and ‘L’

slightly differing in heights are observed. ‘I’ indicates islands close to step edges.b SFM image of the surface of an as-grown YBCO single crystal exhibiting ribbon-like structures ‘R’

(a)

M

P S

C

3 µ m

Fig. 2.

(b)

2 µ m

a Topography of an oxygenated YBCO single crystal measured by SFM. Star-like structures ‘S’ and checkerboard-like structures ‘C’ also present on as-grown crystals are observed on top of the superconduct- ing matrix ‘M’. In contrast to as-grown crystals, particles ‘P’ are present.

b The same crystal after having been stored under ambient conditions for 9 months

face, ranging from some tens of nanometers to a few hundred nanometers in size (Fig. 2a). They are aligned in an orthog- onal network and seem to preferably decorate star-like and checkerboard-like features, which is a further indication of their different chemical nature as compared to the underlying YBCO material. In contrast to the case for as-grown single crystals, rapid etching of the surface of oxygenated YBCO crystals occurs when tunneling currents well above 100 pA are applied during STM measurements in air [11]. Etch- ing typically results in a cascade of unit-cell steps in the YBCO[001] direction.

In Fig. 2b the surface of an oxygenated crystal is shown after it was exposed to the ambient (although protected to some extent from humidity by silica gel) for about 9 months.

The structures mentioned above are still clearly observed, suggesting a slow aging process of the surface. Only the cov- erage of the surface with an orthogonal network of particles has increased substantially. This is an indication that the par- ticles are related to chemical reactions with the ambient: Bar- ium is well known to form BaCO₃ and Ba(OH)2 at YBCO surfaces [13–16] and might therefore be responsible for the formation of the observed particles.

2.2 Surface analysis by ARXPS on an oxygenated crystal After SFM and STM experiments had been performed, ARXPS was used to further analyze YBCO single crystals.

Wide scans and angle-resolved measurements show intrinsic peaks and reveal a 3-nm layer consisting mainly of BaCO3

and CuO. Figure 3 shows two high-resolution spectra of the Ba 4d 3/2 and 5/2 doublet measured at a time interval of 4 months. While the 4d 5/2 components with 87 eV bind- ing energy represent Ba in the superconducting bulk of the single crystals, the 88-eV component originates from insulat- ing YBCO, i.e. surface layers, and the 90-eV component from Ba−CO₃/(OH). The Ba 4d 5/2 line at 87 eV is an excellent indicator of the superconducting YBCO surface [17], since aging is reflected in a decay of the bulk component. In Fig. 3 only a small reduction of this bulk component is observed when both spectra are compared. This implies a good surface

Fig. 3. Two Ba 4d spectra acquired at a time interval of 4 months demon- strate the slow aging rate of the investigated crystal. Ba–1 and Ba–2 denotes YBCO components, whereas Ba–3 is related to Ba–CO3/(OH)xchemisor- bates. The shaded area is subtracted background

quality as well as a very slow aging rate. The formation of an insulating overlayer of thickness d is≈20 times slower than for epitaxial films [17], i.e.

d=k√

t, (1)

where t is the time measured in hours since the start of the aging, and k=0.1 nm/√

h for epitaxial films and k≈ 0.007 nm/√

h for the investigated YBCO single crystal. This dependency (1) is observed for a diffusion-controlled layer growth that is found for thin YBCO films, and is believed to be valid for crystals, too.

2.3 STS at low temperatures (4.6 K)

Differential conductance ( d I/dV ) data acquired in zero field at 4.6 K on a fresh YBCO(001) surface typically exhibit a curve shape like the one shown in Fig. 4. A gap struc- ture and an enhanced conductivity at the gap edges are ob- served. We identify this structure with the superconducting energy gap. Evaluation of the peak-to-peak distance yields

∆pp=16.5 meV. By fitting the data to Dynes’ formula [18], d I

dV ∝ p ε−iΓ

(ε−iΓ )²−∆² , (2)

whereε=EF−eUbias, and∆is the superconducting energy gap, smearing of the gap is taken into account by the broad- ening parameter Γ. The value obtained using this method is usually slightly lower than the peak-to-peak distance and amounts to ∆Dynes=14.3 meV(Γ =4.6 meV). With Tc= 90 K, a reduced energy gap of 2∆/(kBTc)=3.7–4.3 is ob- tained. This is slightly above the BCS weak coupling limit of 3.5. Note the relatively large zero-bias conductance, which amounts to≈70% of the conductance outside the energy gap, i.e.(d I/dV(0 mV)) / (d I/dV(40 mV))≈0.7. This observa- tion has also been reported by Maggio-Aprile et al. [9]. Be- cause d I/dV(0 mV)0, the zero-bias conductance has been added as an offset to d I/dV in (2) for fitting. The large leak- age current and the large broadening parameterΓ =4.6 meV might imply intermediate-state tunneling [19].

Figures 5a,b each show a map of d I/dV at V=0 mV and V=40 mV, respectively, acquired at 4.6 K in a magnetic

dI/dV [arbitrary units]

0,6 0,7 0,8 0,9 1,0 1,1 1,2

-60 -40 -20 0 20 40 60

Data Dynes-Fit

∆pp = 16.5 meV

∆Dynes = 14.3 meV

Bias voltage [mV]

Fig. 4. Differential conductance spectrum obtained as the numerical deriva- tive of measured I(V)characteristics shows the superconducting energy gap.

Peak positions as well as a fit to the Dynes equation are shown

S1222

(a)

20 nm

Fig. 5.

(b)

20 nm

a 80 nm ×80 nm gray-scale representation of d I/dV (0 mV) ac- quired at 4.6 K in a magnetic field of 4 T. Vortices appear as bright spots in the image. b No structure related to vortices is seen in d I/dV (40 mV)

field of 4 T along the [001] direction of YBCO. Brighter pixels indicate a higher differential conductance at that bias voltage. The imaged area is 80 nm×80 nm in both cases.

In Fig. 5a the applied voltage is V =0 mV and thus d I/dV at the Fermi level is displayed. Regions of higher zero bias conductance are seen as bright, oval spots forming a dis- torted lattice. Twelve of these bright spots are observed on the 80 nm×80 nm area. We attribute them to the pres- ence of a vortex lattice, since a calculation of the number of vortices expected in this area at 4 T yields 12.4 (using Φ0=h/2e=2.0678×10⁻¹⁵T m²). This is in good agree- ment with the observed number of bright spots. The mean distance between vortices amounts to 22±2 nm as meas- ured in the fast-scan direction. This is again consistent with the expected inter-vortex distance in a magnetic field of 4 T (24.5 nm for a triangular, 22.7 nm for a quadratic vortex lat- tice). A further proof that the bright spots in Fig. 5a are in fact vortices is the absence of these local variations at energies well above or below the Fermi level. When the applied volt- age lies outside the superconducting energy gap (e.g. 40 mV), no contrast due to the presence of vortices is expected. Fig- ure 5b shows a gray scale representation of d I/dV (40 mV) acquired simultaneously with the data shown in Fig. 5a. No structure can be seen in this image, implying that the DOS at 40 meV above EFdoes not vary locally on this scale.

Note that the elongation of vortices yields an aspect ratio of≈2.5:1. It has been shown that vortex shape asymme- try is due to a/b anisotropy [9, 20]. Since acquiring the data takes≈2 hours in our setup and the aspect ratio is larger than the expected value ofξb/ξa≈1.5, it may also be partly due to drift (either thermal drift of the instrument or drift of the vortex lattice itself).

3 Conclusions

Analyzing the surface of very pure YBCO single crystals grown in BaZrO3 crucibles by SFM and STM shows clean

terraces on as-grown crystals. FFM reveals the presence of minor traces of flux. After oxygenation, particles appear on the crystal surface. This finding is also supported by ARXPS.

Their number per unit area increases over time, and they are believed to originate from the reaction of barium with the ambient. Other surface structures remain unaffected over this period of time. The slow aging rate of such crystals is further confirmed by ARXPS. Analysis of the Ba 4d spectrum shows a good surface quality and a very slow aging of the metal- lic state in these single crystals, that is about 20 times slower than in epitaxial thin films.

At 4.6 K, the superconducting energy gap could be de- termined by using STM/STS, yielding 2∆/(k_BT_c)≈3.7–4.3.

The large zero-bias conductance amounts to ≈70% of the normal conductance outside the gap. The vortex lattice in a magnetic field of 4 T is revealed by analyzing local varia- tions in d I/dV (0 mV).

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