Local characterization of HTS thin films by laser scanning microscopy

Local characterization of HTS thin films by laser scanning microscopy

PHYSICA ELSEVIER Physica C 341-348 (2000) 1435-1438 www.elsevier.nl/Iocate/physc Local Characterization of HTS thin films by Laser Scanning Microsco...

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PHYSICA ELSEVIER

Physica C 341-348 (2000) 1435-1438 www.elsevier.nl/Iocate/physc

Local Characterization of HTS thin films by Laser Scanning Microscopy K. A. Korolev~b, P. M. Shadrin a, J. S. Prestonb, IL A . Hughes b, J.

K.

Namb, V. V. Pavlovskiia

alnstitute of Radioengineering and Electronics of Russian Academy of Sciences, 11 Mokhovaya, 103907 Moscow, Russia bBrockhouse Institute for Materials Research, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4M1 A high-resolution spatially resolved study of electrical inhomogeneties in high-To thin films by Laser Scanning Microscopy has been carded out. A laser beam was focused onto a submicrometer spot on the surface of a thin film microbridge that induced an increase in its local temperature. Bolometric or thermo-electric effects in the heated region resulted in a voltage change, 6V, across the sample that has been measured as a function of the beam position (x,y). Direct measurements of the spatial distribution of the critical current density in the film has been performed from the 5V(x,y) images of HTS microbridges. A system of weak links in the film is observed that may be attributed to the growth island boundaries. A spatial redistribution of the critical current as a function of temperature is observed in this system. It has been demonstrated that for some conditions, the electrical characteristics of the entire microbridge can be determined by only one weak link. The position of the weak links could be observed at room temperature using the thermo-electric response.

1. INTRODUCTION The study of transport in the cuprates is both enriched and complicated by the interplay between crystalline defects and physical processes. Cuprate films inevitably have structural defects due to the lattice mismatch between the cuprates and available substrates. The combination of the extremely short coherence length and the d wave symmetry of the order parameter conspire to make microscopic defects much more relevant to transport than in conventional superconductors. The presence of these defects can directly limit the transport typically through the creation of weak links. Alternatively, the defects can enhance the film's capacity to carry current by providing strong pinning sites for vortices. Finally, defects can significantly modify superconducting parameters such as the magnetic penetration depth and influence the physical processes responsible for dissipation. One approach to understanding the role of defects is to intentionally introduce defects that dominate the global transport properties of the films. For example, this has been carded out for point defects

[1,2] and for large angle grain boundaries [3]. However, to clarify the processes associated with current flow in the high-Tc superconducting samples, a spatially resolved characterization technique is needed. Low-temperature scanning electron microscopy (SEM) [4] and low temperature laser scanning microscopy (LSM) [5] have both been successfully applied to this problem with submicron spatial resolution. In both techniques a local heating of the thin film sample by the focused beam results in an increase of the resistance of this part of the film and changes the total voltage across a current-biased sample. It has also been shown that spatially-resolved measurements of laser-induced voltages in high-To thin films can be carded out, not only at low temperatures, but at temperatures well above Tc (up to room temperature) and even without current bias, using the thermo-electric effects in the samples [6]. Recently, the results of submicrometer electrical imaging of grain boundaries in high-To thin-film junctions by laser scanning microscopy has been presented [7]. In this paper we present our results of electrical imaging of high-T~ thin films together with the high-resolution optical measurements.

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K.A. Koroleu et al./Physica C 341-348 (2000) 1435-1438

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2. E X P E R I M E N T A L

DETAILS

We used 200-nm-thick epitaxial YBCO films, grown on 0.5-ram-thick 6x6 mm 2 LaA103substrates using pulsed laser deposition [8]. Several test structures were fabricated on each chip using a standard photolithographic technique and wet etching in 0.001-M citric acid. Typically, after processing, the bridges exhibited a zero-resistance temperature T~0 w, 90 K, a transition width < 1 K, and a critical current density jc > l0 GA/em2 at 77 K. The R(T) characteristic (typical for all samples) of the microbridge studied is shown in Fig. 1.

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Film Thickness: 200 nm Bridge Width: 6 um B ridge L ength: 100 u m f ,f

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scanning system, thus giving the possibility to change the incident angle of radiation on the objective (5). The samples were mounted on the sample holder of the optical cryostat (6) and were current biased from a separate unit (7). The voltage response AV across the sample after passing it through the amplifier (8) along with the corresponding voltages from the scan oscillators (4) are directed to the computerized data acquisition system (9).

92

94

Temperature (K)

Fig.1. R(T) characteristic of the YBCO microbridge studied. The technique of laser probing is based on the measurements of the voltage change AV across the sample, when the focused laser beam effects the local electrical characteristics of the sample. When the position (x,y) of the focused beam is scanned across the sample, the corresponding twodimensional distribution AV(x,y) of the voltage response is measured and this distribution gives information about the electrical homogeneity of the sample. We used an experimental set-up similar to the one described in detail previously [6], but with improved optical and electronic parts. The schematic presentation of our experimental set-up for laser probing of high-Tc thin films is shown in Fig.2. The optical system is based on a LSM-20 (Zeiss) laserscanning microscope. An Ar-ion laser (1) was used as the source of radiation in these experiments. After the laser beam is expanded (3) it is sent to the x/y scanning system (2). Two scan oscillators (4) supply ac voltages to the galvanometric mirrors of the

Fig.2. A schematic of the experimental set-up for measurements of electrical inhomogeneity of high-To thin fdms by laser probing: 1 - Ar-ion laser, 2- scanning system, 3- beam expander, 4 - scan oscillator, 5- objective, 6- optical cryostat, 7- current-bias circuit, 8- preamplifier and amplifier, 9 - ADC and computer. The high-To samples were mounted on the table of the laser-scanning microscope in the continuousflow optical cryostat. Measurements can be done both at room temperature and at low temperature. The radiation from the Ar-ion laser had a wavelength of 488 nm and a power level of up to 34 mW. The long-distance objective used focused the laser radiation to a spot of approximately 1.2 Ima at the surface of the superconducting sample. The voltage response, AV, of the sample was amplified and recorded as a function of the beam position (x,y) on the sample. High-contrast electrical (AV(x,y)) images consisting of 512 x 512 points with 8 bit signal resolution and spatial resolution of the order of 1 tam were obtained for all types of YBCO thin films under study. The voltage response amplitude is coded by 256 gray colors. For the images presented in this work, the response AV(x,y) is indicated by a color scale in which black corresponds to the minimum negative voltage, white to the maximum

K.A. Koroleo et al./Physica C 341-348 (2000) 1435-1438

positive signal and gray corresponds to zero signal. The reflected and transmitted laser radiation were measured by external radiation detectors so that optical images couid be obtained at the same time as the electric-response images. However, for higher resolution optical images, a halogen lamp with a blue filter and short-distance objectives were used resulting in a resolution of 0.3 lain. To visualize YBCO domains with different twin boundary orientations images were taken in polarized light and compared with images taken with the plane of polarization rotated by 90 degrees. A composite image is obtained through a subtraction of the two polarized images. After this procedure, regions with no polarization contrast become gray (zero intensity), while regions of large polarization contrast become white or black depending on their orientation.

t437

accounts for the significant voltage noise for the 90 K data. As can be seen from the optical image there is a constriction at the central part of the bridge. This constriction corresponds to one intense maximum in bolomelric response. Weak links can be formed not only by internal defects of the film but defects of the shape as well.

I 300 K 91,0K 90.8K 906K 904K

3. RESULTS Optical and electrical images are presented in Fig.3 for an epitaxial YBCO microbridge of 6 ~xn width and 100 gm length. Shown are optical and thermoelectric images at room temperatures and the electrical response with a bias current in the vicinity of Tc. As can be seen from the optical image there is an undercut of approximately 1 gm from both sides of the bridge yielding an effective width of approximately 4 grn. The spatial distribution of the thermo-electric response AV(x,y), induced by local laser heating at zero-bias, is quite inhomogeneous. It has been shown that electrical defects such as grain boundaries produce a thermoelectrical response under LSM probing [6] with odd symmetry. The characteristic length between the inhomogeneities is about 5-10 lain. This is significantly larger than the typical size of the film grains, that are of order of 1 Ixrn. The series of bolometric images in Fig.3c demonstrate a resistive transition in the microbridge at temperatures 91.090.0 K. As one can see, in the normal state (91.0 K), the spatial distribution of the response from the microbridge is nearly homogeneous. As the sample passes through the superconducting transition, we can clearly see that the response to the laser beam is dominated by a few weak links in the film. The amplifier's gain for Fig.3c is changed - 104, 5x104, 105, 105, 2x105, 10 6 from top to bottom. This

90.2K 90.0K Fig.3. (a) The optical image in transmitted light, (b) laser beam induced thermoelectrical images at room temperature and (c) a set of bolometric images at temperatures 91.0, 90.8, 90.6, 90.4, 90.2, and 90.0 K for a 100 pm long YBCO thin film microbridge at a current bias of 55 la.A. The values for the aml31ifier's gain were 104, 5x104 , 105 , 105 , 2x10 and 106 for the bolometric images starting from the upper image. Figure 4 shows the response in the superconducting state at 88 K for different bias currents. The gain used was held constant at 5x10 4 for the upper 7 pictures in which the current varies from 60 to 30 pA. For the next 3 pictures the gain is 2x105 and for the bottom picture it is 10 6. A system of weak links in the film is observed that is likely attributed to growth islands boundaries. It is clear that these weak links correspond to the same features responsible for the response through the transition region. A clear signature that the sample is in the superconducting state is the nonlinear current dependence of the induced response. For example, comparing the images at 60 and 30 ~A, it is clear that a reduction of the bias current by a factor of two changes the response by more than a factor of 4.

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After this procedure the YBCO film appears as a series of dark and bright small spots, apparently arranged randomly.. The size of "element" is around 1 tun, which corresponds to the average size of the growth island of the film as determined by atomic force microscopy. 4. SUMMARY We have applied laser scanning microscopy to the study of current transport through YBCO microbridges. Not surprisingly, we find that the same features responsible for broadening the resistive transition control the critical current below Tc. Polarization contrast measurements at room temperature reveal the grain boundary structure in our films. While grain boundaries are a likely source of weak links in the film, it is clear that not all grain boundaries appear in our bolometric images. Thermoelectric images at room temperature appear to have some correspondence with the bolometric images through the transition region and below Tc. REFERENCES

Fig.5. The result of a subtraction of two optical images with different polarizations.

The film's internal domain structure is shown in Fig.5 using the polarization contrast between two optical images taken at room temperature. The microbridge is oriented horizontally at the center of the image. The surrounding substrate appears as a gray plane (no polarization contrast) with some defects that show some polarization dependence in their light scattering. The large triangular area on the left side of the bridge is the metallic contact pad.

[1] S.H. Moffat, 1LA. Hughes and J.S. Preston, Phys. Rev. B55 R14741 (1997). [2] A.E. White, K.T.Short, R.C. Dynes, A.F.J. Levi, M. Anzlowar, K.W. Baldwin, P.A. Polakos, T.A. Fulton and L.N. Dunkleberger, Appl. Phys. Lett. 53, 1010 (1988) [3] D.G. Steel, J.D. Hettinger, F. Yuan, D.J. Miller K.E. Gray, J.H. Kang, J. Talvacchio, Appl. Phys. Lett. 68, 120 (1996) [4] R. Gross, T. Doderer, R. P. Huebener, et al., Physica B 169 415 (1991). [5] Yu. Ya. Divin, F. Ya. Nad', V. Ya. Pokrovski, P. M. Shadrin, IEEE Trans. Magn. 27 1101 (1991). [6] Yu. Ya. Divin, P. M. Shadfin, Physica C 232 257 (1994). [7] P. M. Shadrin, Yu. Ya. Divin, Physica C 297 69 (1998). [8] A. W. McConnell, R. A. Hughes, A. Dabkowski, H.A. Dabkowska, J.S. Preston, J.E. Greedan, T.Timusk, Physica C 225 7 (1994).