Film growth and roughness of YBCO thin films shown by X-ray reflection and RHEED

ELSEVIER

Physica C 277 (1997) 243-251

Film growth and roughness of YBCO thin films shown by X-ray reflection and RHEED Ataru Ichinose, Akita Shirabe Central Research Institute of Electric Power Indu~try, Komae Research Laboratory, 2-11-1, lwato Kita, Komae, Tokyo 201, Japan Received 20 December 1997

Abstract

YBCO thin films were prepared on different kinds of single-crystal substrates by two film growth methods with evaporation deposition: i.e., layer-by-layer growth and block-by-block growth. In layer-by-layer growth, we observed the oscillation of Reflection High Energy Electron Diffraction (RHEED) intensity, representing unit cell growth. In block-byblock growth, though we were unable to observe RHEED intensity oscillation, film growth was speculated by RHEED patterns and XRD data. We also measured X-ray reflection data for various films prepared on different substrates by the two film growth methods. The reflection data was collected for incident beam angles of 0.5 to 1.5°. The roughness of the film surface and film/substrate interface was analyzed by X-ray reflectivity theory. We discuss the film growth on different substrates. Furthermore, the relation between film roughness and film growth method will also be discussed. Keywords: Thin film; X-ray reflectivity; RHEED

1. Introduction

High-T¢ superconducting films have been studied in numerous groups because of their key role in technology not only for electronic applications, but also for large-scale applications such as power transmission cables. Problems such as rough surface morphology, poor crystal quality, and lack of crystal control have made their fabrication difficult. Several film growth techniques, atomic layer-by-layer growth, layer-by-layer (unit-by-uni0 growth, and block-by-block growth, have been studied. The crystal growth of films has been analyzed by Reflection High Energy Electron Diffraction (RHEED) in a comparatively high vacuum. RHEED is a useful method for characterizing the surface structure and controlling the crystal growth on an atomic scale.

The layer-by-layer growth of a YBCO film using a co-evaporation technique and the correspondence of one RHEED oscillation period to the layer-by-layer growth of the minimum crystal unit cell have been reported [1]. Therefore, the unit cell layer-by-layer growth can be controlled by observing the oscillation of RHEED intensity during deposition. The surface morphology was deduced from the RHEED pattern. An atomic layer-by-layer growth is usually studied with the MBE technique. This growth technique supplies a monolayer (a metal oxide layer) in sequence, for example, / B a O / C u O 2 / Y / C u O 2 / B a O / C u O / . Locquet et. al. [2,3] have reported that in the above deposition sequence, the first two monolayers, BaO and CuO, have probably reacted before the subsequent deposition of Y203. From a thermodynamic point of view, they deposited the blocks

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sequentially, for example, 2 monolayers of BaO, 1 monolayer of Y203, and then 3 monolayers of CuO, following certain rules. They called it block-by-block growth. The surface morphology of superconducting films is usually elucidated using Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) or some other equipment. AFM can analyze the roughness of a unit cell height within a small area of about 1 /~m × I /xm. Though by conventional SEM some particles were observed on the film surface on a/zm scale, it was difficult to observe smaller particles or a smoother surface. These methods reveal the atomic or particle images of the film surface. On the other hand, X-ray reflection cannot obtain images of surface roughness. In X-ray reflection, the roughness information was obtained as the mean value over the region on which the X-ray was incident. The region is of the order of about a few mm or cm, because of the low angle of incidence of the X-ray. Furthermore, the analysis using X-ray reflection yields some information on the film thickness and the roughness of the surface and interface. We prepared the YBCO superconducting thin films on different kinds of single crystal substrates using two methods of crystal growth, layer-by-layer (unit-by-unit) growth and block-by-block growth. We measured the X-ray reflection of these films. We discuss the film growth on different kinds of single crystal substrates using X-ray reflection data and RHEED data.

2. Experimental procedures The YBCO superconducting films were prepared on the single crystals SrTiO3(100), MgO(100) and

LaA103(100) by layer-by-layer (unit-by-unit) growth or block-by-block growth. Pure oxygen gas for oxidation was introduced near the substrate at a mass flow rate of 22 cm3/min. The films were deposited at several temperatures. Typical deposition temperatures in the case of the SrTiO 3, MgO, and LaA103 substrates were 860, 830 and 890°C, respectively. Each deposition temperature is optimized with regard to the phase stability and the superconductivity. The temperature was measured at the heater side of the substrate holder. The background pressure during deposition was about 1.0 × 10 -4 torr. In layer-bylayer growth, we used the co-evaporation deposition technique. The evaporation speed of three independent metal evaporation sources, Y, Ba and Cu, was controlled by monitoring the evaporation speed using a quartz crystal oscillator. The film growth speed was about 0.4 A/s. For sample 1, the YBCO film was grown on a SrTiO 3 substrate by the block-byblock method. For sample 4, the first layer was grown by block-by-block and then the 19 layers by layer-by-layer on the SrTiO 3 substrate. Samples 2 and 5 were grown on MgO substrate by block-byblock and layer-by-layer, respectively. Samples 3 and 6 were grown on a LaAIO 3 substrate by blockby-block and layer-by-layer, respectively. The deposition conditions are summarized in Table 1. The two temperatures in Table 1 represent the temperature change during deposition. We also observed the RHEED patterns and the oscillation of RHEED intensity during film deposition. The incident direction of electrons is along the a- or b-axis of YBCO. In block-by-block growth, each of the three metals was deposited in sequence, with flowing pure oxygen gas. The block-by-block growth speed was reduced to about 1/3, because the evaporation speed of each source was nearly the

Table 1 Film growth lmrameters. * composition differs from the nominal composition of YBa2Cu3Oy. Block and Layer means block-by-block growth and layer-by-layer growth, respectively No.

Substrate

Temperature (*(2)

Film growth

1 2 3 4 5 6

SrTiO 3 MgO LaAIO 3 SrTiO 3 MgO LaAIO 3

860 780 - , 830 840 ~ 890 860 830 890

1* Block 1 * Block 1 * Block 1 ' Block 20 Layer 20 Layer

& & & &

19 Block 19 Block 19 Block 19 Layer

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same in both film growth methods. The supplied oxide metals diffused and reacted on the substrate and then the YBCO crystal structure was formed. Although we observed RHEED patterns and intensity of RHEED, we were not able to observe the oscillation of RHEED intensity representing the unit cell growth. X-ray reflection measurements were performed using an X-ray diffractometer (Mac Science: MXP18) equipped with a flat graphite monochromator. CuK a radiation was used. All of the prepared films were observed with 001 peaks only and they had a c-axis orientation. The c-axis orientation was also confirmed by a pole-figure measurement. Grazing incidence X-ray reflectivity was measured from 1° to 2.5 ° with a step width of 0.01 ° in 20.

3. Theory and analysis of X-ray reflectivity [4,5] The theories of X-ray reflection from surfaces were developed by many researchers, and Parratt showed a simpler treatment. For simplicity, a threelayered model including air (consisting of air, a film

and a substrate) is used to fit the reflection data. The reflectivity is given by R=

F 1,2 2 + aFI,2F2,3 + F 2,3 2 F 1,2 2 F 2,3 2 ' 1 - I - a F 1,2F2.3 +

where the subscripts 1, 2 and 3 indicate air, film and substrate, respectively and FI, 2 and F2, 3 are the amplitude reflectivity at each interface, given by F,,2 = ( f l - f 2 ) / ( f l

+f2),

F2.3 ----( f 2 - - f 3 ) / ( f 2

+f3)'

where, for the critical angle 0c(n), n = 2 or 3,

f.= [O- 02(n)]'/2, and for a layer thickness d a = 2 cos(41rf2d/A ) .

The values of the critical angle are determined from the mass density (electron density is exact) assuming homogeneous film growth. In the case 0 :~ 0c, we get the expressions for Fi, 2 and F2,3, 02(2) 02(3) - 0c2(2) F1,2 = 402 ' F2,3 = 402

Fig. 1. RHEED patterns of two early sequences on the SrTiO3 substrate in block-by-block growth. (a) after Ba deposition on substrate; (b) after Y deposition on Ba oxide layer, (c) after Cu deposition on Y/Ba oxide layers and it is the end of the first deposition sequence; (d) after Ba deposition in the second sequence; (e) after Y deposition; (f) after Cu deposition and it is the end of the second deposition sequence.

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The above theory assumes perfectly flat interfaces. The roughness of a surface or an interface is most simply incorporated by assuming a Gaussian form, where each interface reflectivity is multiplied by the factor exp(-½[(4~'o'n/A )

sin ~/]2},

where o-n is the root-mean-square value of roughness. tr I and tr 2 express the roughness of the 1-2 interface (between air and the YBCO film), and the 2-3 interface (between the YBCO film and substrate), respectively. The variable parameters of the theory are fitted to the data from 1° to 2.3 ° in 20 using a least-squares fit. Data for which the intensity of the reflection is smaller than about 300 cps are excluded from X-ray reflectivity refinement to eliminate background noise. The R D factor of the refinement is calculated using R D = Y'.log [ Ii(o) - l i ( c ) [ / ~ l o g

li(o ) × 100,

where li(o) is the observed intensity of reflection at the point i and ;i(c) is the calculated intensity at the same point.

4. Results

and discussion

4.1. RHEED and film growth 4.1.1. Block-by-block growth The deposition sequence in block-by-block growth is B a / Y / C u during oxygen gas flow. The RHEED patterns for sample 1 in the first two deposition sequences on SrTiO 3 are shown in Fig. 1. The RHEED patterns of the surface deposited Ba or Y are nearly the same, and only the pattern intensity decreases. This indicates that Ba and Y do not react. When Cu is deposited, at first the intensity becomes lower, then bright streaks appear after Cu is sufficiently supplied. In the second sequence, although some spots appeared on the streaks by Ba deposition, the RHEED pattern changes to sharp streaks again after Cu deposition. The change of the RHEED pattern during one sequence is repeated after the third sequence. We consider that the YBCO structure grows smoothly and homogeneously. The RHEED patterns for sample 2 do not change with Y deposition on the BaO layer on the MgO

Fig. 2. RHEED patterns of two early sequences on the MgO substrate in block-by-block growth. (a) after Ba deposition on substrate; (b) after Y depositionon Ba oxide layer;,(c) after Cu depositionon Y/Ba oxide layersand it is the end of the first depositionsequence;(d) after Ba deposition in the second sequence;(e) after Y deposition; (f) after Cu deposition and it is the end of the second deposition sequence.

A. lchinose, A. Shirabe/ Physica C 277 (1997)243-25l

substrate. This tendency is the same as with the SrTiO 3 substrate. The RHEED pattern for sample 2 changes drastically, before and after the first Cu deposition on the MgO substrate. In the second sequence, the RHEED pattern after Ba deposition is observed with bright streaks and some obvious spots on the streaks appear. After several sequences, some streaks reappear in the RHEED pattern. This indicates that in the early sequence the YBCO structure does not grow smoothly at the interface, and after that the YBCO structure grows with atomic smoothness. Only the intensity of the RHEED pattern decreases remarkably for sample 3, deposited with Ba or Y on the LaA10 3 subslrate, and the observed streaks are weak. After Cu deposition, the intensity of streaks recovers. At the end of the second sequence, some spots appear and the streaks become weak. The improvement of the streak intensity in the RHEED pattern for sample 3 is not observed during deposition. It is completely different from sample 2 (MgO substrate), and indicates that the film does not grow in epitaxy. The RHEED patterns for samples 2 and 3 of the first two sequences on the MgO and

247

LaA10 3 substrates are shown in Figs. 2 and 3, respectively. 4.1.2. Layer-by-layer growth

The first layer of sample 4 is deposited with a Ba-rich composition using block-by-block growth. The nominal composition of YBCO is deposited using layer-by-layer growth after that. The RHEED oscillations are observed during deposition. Though the oscillation amplitude is very weak, the oscillation waveform does not change. We interpret the observation of the RHEED oscillation as layer-by-layer growth. After the first layer is deposited on the MgO substrate, the intensity of the RHEED pattern for sample 5 decreases remarkably and the RHEED oscillations are not observed clearly. After the second layer deposition, the intensity decreases slightly and an obvious oscillation is gradually observed. On the other hand, in the early part when a few layers are deposited on the LaA10 3 substrate, the oscillations for sample 6 are observed clearly, with a period of about 30 s. After that the intensity decreases immediately and the oscillations are not observed

Fig. 3. RHEEDpatternsof two early sequenceson the L a A I O 3 substratein block-by-blockgrowth. (a) after Ba depositionon substrate;(b) after Y depositionon Ba oxide layer;,(c) after Cu depositionon Y/Ba oxide layers and it is the end of the fwst deposition sequence;(d) after Ba deposition in the second sequence;(e) after Y deposition;(f) after Cu deposition and it is the end of the second deposition sequence.

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r

I

k Fig. 4. Final RHEED patterns in block-by-block growth and layer-by-layer growth. (a-c) on SrTiO3, MgO and LaAIO 3 substrate by block-by-block growth, respectively; ( d - 0 on SrTiO3, MgO and LaAIO3 substrate by layer-by-layer growth, respectively.

clearly. The RHEED patterns of the films deposited on the MgO and LaAIO 3 substrates do not change much during deposition. The final RHEED patterns for each film are shown in Fig. 4.

4.2. X-ray reflectivity refinement and film growth Fig. 5 shows the X-ray reflection patterns and the final calculated patterns for each sample. Table 2 lists the final refinement parameters. The ref'mements for samples 2, 3, 4 and 6 are carded out by limiting data points because of the weak reflection intensity. The weak intensity may be due to the fact that (1)

the surface is not smooth or (2) the interface is not smooth.

4.2.1. Block-by-block growth The g D factor for sample 1 is 0.17%. This small value means that the refinement model agrees with the present film growoth model. The calculated film thickness, d =-249.3 A, is almost the same as the expected thickness, 230-240/~. Both values of cr 1 and tr 2 are about 18 /~. As a result, we conclude that the ~rBCO film for sample 1 grows atomically smoothly and homogeneously. This result is consistent with the RHEED result. The value of tr2 for sample 2 deposited on the

Table 2 Final resulls of X-ray reflectivity refinemeat. Figures in parentheses are estimated standard deviations to the last significant digit No.

Subslrate

d (~)

or I (,~)

or 2 (A)

R D (¢~)

Data points

1 2 3 4 5 6

SrTiO3 MgO L,aAIO3 SrTiO 3 MgO LaAIO3

249.3(9) 182(3) 150.1(6) 210.9(7) 235.2(6) 263(2)

17.82(15) 17.9(4) 26.4(2) 21.8(2) 17.3(2) 23.5(4)

18.1(4) 22.9(10) 22.7(3) 19.7(3) 17.5(3) 23.9(7)

0.17 0.38 0.23 0.31 0.32 0.37

130 95 68 112 130 72

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MgO substrate is very large and the standard deviation is fairly large. The tr 2 values of sample 2 using block-by-block growth and of sample 5 using layer-

6

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Fig. 5. X-ray reflection pattems for each sample. Plus marks show observed X-ray reflection intensifies and a solid line represents calculated intensities. (a-c) on SrTiO 3, MgO and LaAIO 3 substrate by block-by-block growth, respectively. (d-f) on SrTiO 3, MgO and LaA10 3 substrate by layer-by-layer growth, respectively.

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A. lchinose, A. Shirabe / Physica C 277 (1997) 243-251

at the interface or the lack of growth of the homogeneous film, because the surface roughness of each substrate is almost the same. Furthermore, the amplitude of reflectivity is dependent on the electron density difference between the film and substrate. The electron density difference between YBCO and the LaAIO 3, SrTiO 3, and MgO substrates increases in order. The reflectivity amplitude of sample 5 (MgO substrate) is clearly high, while that of samples 3 and 6 (LaAIO 3 substrate) is very small. In spite of the MgO substrate, the amplitude for sample 2 is very small. This means that the electron density changes continuously at the interface in sample 2, and is consistent with the above result obtained from the large difference of interface roughness. We conclude that the intermediate layer, that is the layer with different electron density from the substrate or the YBCO films, grows at the interface in sample 2. Therefore, the R o factor for sample 2 is fairly large at 0.38%. The interface roughness for sample 3 and sample 6 is similar in film growth and many twin structures are observed on the substrate-deposited film. As a result, the surface of the LaAIO 3 substrate probably becomes rough with a high temperature treatment. Samples 2 and 3 have a large difference between or 1 and or 2. For sample 2, or 1 < or 2. The reason for the large o-2 value in sample 2 has already been explained above. Because of the observation of some streaks in the final RHEED pattern for sample 2, or 1 should be similar to that of another film for which some streaks are observed, i.e. sample 5. or 1 for samples 2 and 4 are 17.9(4) A and 17.3(2) ,~, respectively, or 1 for sample 3 is remarkably larger than or 2. The final RHEED pattern has some obvious spots which indicate a rough surface as shown in Fig. 4(c). As a result, the tendency of the or parameter change is consistent with the film roughness expected from the RHEED pattern. The thickness of samples 2 and 3 with MgO and LaA10 3 deposited on the substrate are smaller than the expected film thickness, about 230-240 ,~. The standard deviation of the thickness, d, for sample 2 is large and is interpreted as film growth with different electron density at the interface. As a result, the thickness is estimated as slightly thicker than the expected thickness. On the other hand, the standard deviation of the thickness, d, for sample 3 is not so

large. Sample 3 does not grow in epitaxy, according to RHEED, so we conclude that this is the main cause for the smaller thickness. 4.2.2. Layer-by-layer growth Although the calculated film thickness, d, is slightly smaller and values of both or 1 and or 2 are slightly larger in sample 4, there are no parameters which have an unusual value or a large deviation. We conclude that the surface roughness and homogeneity of sample 4 to be fairly good. The calculated film thickness, d, in sample 5 is almost the same as the expected thickness, 230-240 ,~. Values of both o- 1 and or 2 in sample 5 are about 17-17.5 ~,. As a result, the YBCO film for sample 5 grows atomically smoothly and homogeneously. This result is consistent with the RHEED result. The film thickness for sample 6 is larger than the expected thickness and the standard deviation is very large. If the cause of the large standard deviation in sample 6 is the same as for sample 2, we conclude that the La ions diffuse into the YBCO film and the electric density increases at the interface. The early deposition temperature in sample 3 is 840°C, while the one in sample 6 is 50°C higher at 890°C. We conclude that the La ions diffuse more easily into the film rather than in sample 3.

5. Summary We prepared YBCO films on SrTiO3(100), MgO(100) and LaAIO3(100) substrates using layerby-layer growth and block-by-block growth. The RHEED pattern and intensity were observed during deposition. The X-ray reflectivity was also measured and the film thickness, the roughness of film and interface were analyzed by the theory of reflectivity. We estimated the film growth on various kinds of substrate using both the RHEED results and the results of X-ray reflectivity analyses. The refinement results of X-ray reflectivity were consistent with the RHEED results. It was found that the X-ray reflectivity analyses are a useful method to understand film growth. We found out the following by means of the X-ray reflectivity analyses. In the case of the SrTiO 3 substrate, the films grew homogeneously and smoothly for both film growth methods, block-by-

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block growth and layer-by-layer growth. In the case of the MgO substrate, the intermediate layer grew at the interface in block-by-block growth, while the films grew homogeneously in layer-by-layer growth. In the case of the LaA103 substrate, we did not observe the intermediate layer at the interface in block-by-block growth.

Acknowledgments We thank Shigeo Sunaga of Techno Service Co., Ltd. for his help in film preparation and also the colleagues of CRIEPI for their helpful discussions.

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