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Influence of MgO surface conditions on the in-plane crystal orientation and critical current density of epitaxial YBCO films
Physica C 400 (2004) 143–152 www.elsevier.com/locate/physc
Influence of MgO surface conditions on the in-plane crystal orientation and critical current density of epitaxial YBCO films J. Du *, S. Gnanarajan, A. Bendavid CSIRO Telecommunications and Industrial Physics, P.O. Box 218, Lindfield, NSW 2070, Australia Received 7 July 2003; accepted 24 July 2003
Abstract The effect of magnesium oxide (MgO) surface conditions on in-plane grain orientation and critical current density of epitaxial YBa2 Cu3 O7 (YBCO) films was systematically investigated. The MgO substrates were either ‘‘as received’’ or stored for some time, cleaned using different methods and lithographically prepared for our step-edge junction devices. The YBCO films were grown via reactive thermal co-evaporation by Theva, GmbH. The surface characterisation of MgO substrates was studied using X-ray photoelectron spectroscopy (XPS). The in-plane grain orientation of the YBCO films was studied by means of X-ray diffraction (XRD) /-scan and the critical current density was measured for the XRD scanned samples. The surface condition of the MgO substrates was found to have a strong influence on the inplane grain orientation and the critical current density of the YBCO films. The MgO substrates with a degraded or contaminated surface gave rise to 45 grain misorientation in YBCO films and reduced the critical current density. A final process step using a low energy Ar ion beam etching (IBE) of the MgO substrates prior to the YBCO film deposition was found effective in removing the in-plane grain misorientation and promoting the growth of perfectly aligned c-axis YBCO films. 2003 Elsevier B.V. All rights reserved. PACS: 74.76.Bz; 81.65.Cf Keywords: YBa2 Cu3 O7 ; Magnesium oxide (MgO); Grain orientation; Ion beam etch; XRD phi-scan; X-ray photoelectron spectroscopy
1. Introduction Single-crystal magnesium oxide (MgO) is one of the popular substrates for the growth of high
*
Corresponding author. Tel.: +61-2-9413-7107; fax: +61-29413-7161. E-mail address: [email protected] (J. Du).
quality YBa2 Cu3 Ox (YBCO) thin films due to its low dielectric loss, thermal expansion coefficient similar to that of YBCO, and low cost despite the large lattice mismatch with YBCO of 9% [1]. YBCO (1 0 0) films were grown on MgO (1 0 0) substrates for our step-edge junction superconducting quantum interference devices (SQUIDs) and other junction based devices. Good control of the Josephson junction parameters requires a
0921-4534/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2003.07.005
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reproducible and high quality growth of YBCO films. Deviation from the required specification of the film properties usually results in either nonworking devices or a need for post-trimming the devices [2,3]. Step-edge patterns were routinely prepared on MgO substrates using a standard lithographic process and ion milling technique prior to the deposition of the YBCO thin films. This lithographic processing of a MgO substrate was found to have a detrimental effect on the surface quality and, consequently, the quality of the epitaxial YBCO film. Similarly, exposure of MgO substrates to the environment or even being stored in dry nitrogen for a period of time also has an adverse effect on the quality of the YBCO films. The deterioration of MgO substrates by exposure to the environment or during processing is a major disadvantage for YBCO thin film growth. In-plane grain misorientation has been observed in YBCO films grown on MgO (1 0 0) substrates by other groups [1,4–8]. Grain misorientation varied in angle [1,4,7], and could be affected by MgO surface polishing techniques [4,5] and by YBCO film deposition techniques and conditions [1,6,8]. Annealing of MgO substrates at high temperatures prior to film deposition has been found to be useful in improving in-plane grain orientation in the YBCO films [4,6,9]. In our YBCO (1 0 0)/MgO (1 0 0) films, the grains were found to be either perfectly oriented or 45 misoriented. The fraction of the 45 misoriented grains in the film was found to be strongly associated with the surface condition of the MgO substrate due to the preparation method. The existence of 45 misoriented grains caused the film Jc to drop significantly and reduced our device yield. Although, there is some discussion in the literature on the growth mechanism of misoriented grains and the dependency of film Jc on the misorientation angle, there is, however, a lack of a systematic study on the relationship of the grain misorientation in YBCO films and the MgO surface quality as result of different substrate preparation methods. We found that various degrees of 45 in-plane misorientation could be resulted in the same batch of YBCO films grown on the same batch of MgO substrates due to different substrate cleaning and processing methods. There is also a lack of experimental data in the literature
showing the relationship of film Jc with the fraction of 45 misoriented grains for a broad range of Jc values. In order to control the quality of YBCO films and the yield of devices, we need to understand the cause of the 45 grain misorientation observed in our YBCO/MgO (1 0 0) films and its effect on the critical current density. Furthermore, we need to develop a cleaning process to improve the MgO surface quality after being lithographically processed or stored for a period of time prior to the film deposition. In this work, we systematically investigated the influence of various surface conditions of MgO on the in-plane grain orientation and the critical current density of the YBCO films. The different surface conditions of MgO substrates considered were due to various preparation methods. We describe a new process to clean the MgO surface to ensure the growth of perfectly grain-aligned YBCO films.
2. Experimental Single-crystal MgO (1 0 0) substrates of 10 mm · 10 mm · 0.5 mm size were supplied by ESCETE, Holland, and Crystal GmbH, Germany. In our experience, commercially available MgO substrates varied in quality from supplier to supplier and from batch to batch in terms of surface defects, roughness [10] and contaminants. We usually screen out the substrates with excessive polish/scratch lines and other visual defects by using an optical microscope at ·1000 magnification. To reduce possible variables in the substrates and the films, we compared the substrates from the same batch of the same supplier and prepared with our standard washing and processing methods. MgO samples were prepared in different ways: ‘‘as received’’ without treatment, washed in detergent or solvents, exposed to the laboratory environment or stored in a dry nitrogen environment for a period of time, and lithographically prepared with or without a final Ar ion beam etching (IBE), as summarised in Table 1. In the standard washing process, the sample was either washed in Teepol detergent, diluted with deionised (DI) water in 4:1 ratio, in an ultrasonic bath for 2 min, followed
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Table 1 Information about the MgO substrates and properties of the resulting YBCO films Sample ID
MgO substrate treatment
Fraction of 45 grains in YBCO
YBCO film Jc (MA/cm2 )
Aa Ba
‘‘As received’’ without any treatment Washed in diluted Teepol detergent in ultrasonic bath, followed by DI water rinse and N2 blow dry IBE cleaned for 8 min at VB ¼ 300 V Fresh MgO After being stored in dry N2 cabinet for 20 months IBE (VB ¼ 300 V) cleaned after being stored for 20 months in dry N2 Lithographically patterned without a final IBE Lithographically patterned with a final IBE at VB ¼ 500 V Lithographically patterned with a final IBE at VB ¼ 300 V Washed in acetone in ultrasonic bath, followed by ethanol rinse and N2 blow dry Exposed to laboratory environment for 6 weeks
0.30 0.01
0.005 2.0
Ca Db Eb Fb Gc Hc Ic Ja Kc
0.00 0.00 0.20 0.00 0.11 0.02 0.00
2.4 3.0 0.22 2.4 1.1 1.9 2.2
a
MgO substrates of a same batch from Crystal. MgO substrates of a same batch from ESCETE. c MgO substrates of a same batch from Crystal. b
by a deionised water rinse for less than 1 min and blown dry with nitrogen gas, or washed in acetone in an ultrasonic bath, followed by ethanol rinse and blown dry with nitrogen gas. Teepol detergent is a secondary alcohol sulphate and is regularly used to clean various substrates. The lithographically processed MgO substrates were prepared using our standard processing steps for step-edge patterns [11] which include photoresist (PR) spuncoating, UV exposure, PR development, the IBE etching at a non-normal angle, washing in acetone and ethanol, and nitrogen blow dry. The final IBE cleaning process involves refreshing the MgO substrate by etching away a thin surface layer using Ar ion IBE at a relatively low energy (e.g. 300 eV) and at an Ar pressure of 63 · 104 mbar, prior to the YBCO film deposition. YBCO films of 200–250 nm thicknesses were deposited on the MgO substrates by THEVA GmbH, Germany using a reactive thermal coevaporation technique [12]. All samples except for one used in this investigation were deposited in two batches. Therefore, possible variations due to film deposition procedures were minimised. There is no observable misorientation in the c-axis for these films as determined by XRD h–2h measurements and grain misorientation only occurred in the ab plane. The critical current density, Jc ,
of the perfectly grain-oriented films was in the range of 2–3 MA/cm2 which was desired for our SQUIDs. In-plane growth orientation of the YBCO films was investigated by X-ray diffraction (XRD) /scans using a Philips XÕPert diffraction system and with the Cu-Ka radiation operated at 45 kV and 40 mA. The scan was performed on the YBCO (1 0 3) plane which has a 45 angle orientation to the substrate plane. The volume fraction of the misoriented grains was determined from the relative peak intensities. The chemical composition of the surface of the MgO substrates was assessed by X-ray photoelectron spectroscopy (XPS) using a Specs (SAGE 150) photoelectron spectrometer operated with an Mg X-ray source. The binding energies of the photoelectron peaks from the films were referenced to the C 1s peak at 284.6 eV to compensate for any surface charging. The critical current density, Jc , of YBCO films at 77 K was determined either using an inductive Jc measurement by THEVA or an in-house standard 4-point transport current–voltage (I–V ) measurement. In the transport current method, microbridges of width 50 or 100 lm were patterned and the critical current, Ic , of the microbridges was measured. Ic was determined from the onset of a nonzero voltage of I–V curve using a 1 lV criterion.
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Film Jc was given by Ic =A, where A is the crosssectional area of the microbridge.
3. Results 3.1. In-plane orientation of YBCO films
counts/s
60k 50k 40k 30k 20k 10k 0
(b) sample B (detergent washed)
counts/s
70k 60k 50k 40k 30k 20k 10k 0 70k 60k 50k 40k 30k 20k 10k 0
(c) sample C (IBE cleaned)
counts/s
XRD /-scan analysis has been undertaken on a large number of YBCO films. The results shown here are typical for those of similar substrate conditions. Information about the MgO substrates used in this paper and the /-scan results of the YBCO films grown on these substrates are summarised in Table 1. Fig. 1 compares the /-scan spectra of the YBCO films on the MgO substrates of the same batch (Crystal): (a) ‘‘as received’’ without washing (sample A), (b) washed in deter-
(a) sample A ( "as received")
0
50
100
150
200
250
300
350
phi (deg) Fig. 1. XRD /-scan spectra for the YBCO films grown on the MgO substrates of (a) ‘‘as received’’ (sample A), (b) washed in detergent (sample B), and (c) IBE (300 V) cleaned (sample C).
gent (sample B) and (c) IBE (300 V) cleaned (sample C). The peaks at 0, 90, 180 and 360 come from the perfectly aligned grains (we denote them as 0 rotation). The fourfold symmetry reflects the cubic structures and the similar values of the a- and b-axes. The 45 peaks represent a 45 rotation of some YBCO grains in relationship to the substrate, i.e. the a-axis of the film [1 0 0] is parallel to the [1 1 0] direction rather than [1 0 0] direction of the substrate. Strong 45 misorientation peaks are clearly shown for the film grown on the ‘‘as received’’ substrate (trace (a)). Only slight 45 peaks were visible for the washed substrate (trace (b)) suggesting a substantial improvement in in-plane crystal orientation of the epitaxial film. IBE cleaning of the substrate completely eliminated the misorientation peaks (trace (c)) indicating a perfectly grain-aligned YBCO film. Storage of MgO substrates was found to affect the surface quality and therefore the quality of epitaxial YBCO films. Fig. 2 shows the /-scan spectra of YBCO films on another batch MgO substrates (ESCETE); (a) ‘‘fresh’’ substrate (sample D), (b) after being stored in dry nitrogen cabinet for 20 months (sample E) with some short exposures to the environment when taking out other substrates, and (c) IBE etched after being stored for 20 months (sample F). Films on substrates E and F were deposited together and the film on substrate D was deposited 20 months earlier. A perfectly aligned YBCO film was obtained from the fresh substrate (trace (a)), but 45 peaks were observed in the film grown on the 20month old MgO (trace (b)). This shows that storage of MgO has some influence on YBCO film quality due to degradation of the MgO surface. However, our experiment has shown that the degradation of MgO is rather slow in a dry nitrogen environment compared to that exposed in the laboratory environment. The film grown on the IBE etched old substrate (trace (c)) showed no misorientation peaks suggesting a perfectly aligned YBCO film. The implication is that IBE etching can be used to refresh the degraded MgO substrate due to storage. Fig. 3 compares the /-scan results on the YBCO films grown on (a) patterned MgO substrate without a final IBE etching process (sample G),
J. Du et al. / Physica C 400 (2004) 143–152
counts
70k 60k 50k 40k 30k 20k 10k 0
counts
80k 70k 60k 50k 40k 30k 20k 10k 0
counts
(a) sample D ("fresh")
100k
80k 70k 60k 50k 40k 30k 20k 10k 0
counts
80k 60k 40k 20k
counts
0 80k 70k 60k 50k 40k 30k 20k 10k 0
(b) sample E (20 mons old)
100k
counts
(a) sample G (patterned, no final IBE)
(b) sample H (patterned, IBE at 500V)
(c) sample F (20 mons old, IBE)
(c) sample I (patterned, IBE at 300V)
80k 60k 40k 20k 0 0
50
100
150
200
250
300
350
phi (deg) Fig. 2. XRD /-scan spectra for the YBCO films grown on the MgO substrates of (a) fresh (sample D), (b) 20-month old (sample E), and (c) 20-month old but IBE etched (sample F). These three substrates were from the same batch. The films of (b) and (c) were deposited together.
(b) patterned MgO substrate with a final IBE etching at 500 V for 8 min (sample H), and (c) with a final IBE etching process at 300 V for 8 min (sample I). The three samples went through the same lithographic processes before the final IBE cleaning and the films were deposited together. The results showed that the lithographic process degrades the MgO surface and results in the growth of 45 misoriented YBCO grains. A final IBE process was shown to be effective in removing the misoriented grains as shown in (b) and (c). However, IBE at higher energies (e.g. 500 eV) can affect the in-plane grain orientation slightly as indicated by the weak 45 peaks in Fig. 3(b). It could be due to minor Ar contamination or implantation on the surface as probed by XPS or possible surface defects caused by ion bombardment. In most cases, the misorientation peaks for the ion-etched
147
0
50
100
150 200 phi (deg)
250
300
350
Fig. 3. XRD /-scan spectra for the YBCO films grown on the MgO substrates of (a) patterned without a final IBE (sample G), (b) patterned with a final IBE at 500 V for 8 min (sample H), and (c) with a final IBE at 300 V for 8 min (sample I).
samples were negligible. A perfectly grain-oriented film was obtained from the low energy (300 eV) IBE cleaned MgO substrate indicated by 0 only peaks in trace (c). Results here suggest that the final IBE process of the patterned MgO substrates at a low energy level prior to the film deposition can be used to remove the contaminated or degenerated surface layer due to lithographic processing. The final IBE cleaning of the patterned MgO substrates prior to YBCO film deposition has become a standard procedure for fabricating our step-edge junction devices. 3.2. Critical current density Jc Critical current density, Jc , of the YBCO films grown on the MgO substrates was assessed for the
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3.3. Surface characterisation of MgO by XPS XPS was used to investigate the surface of the MgO substrates that have been prepared or cleaned in various methods. The major differences in the XPS spectra were found in the carbon (C 1s) and oxygen (O 1s) peaks. Figs. 5 and 6 compare the XPS spectra of C 1s and O 1s for four MgO
2.5
YBCO film J c (MA/cm )2
films with different fractions of 45 in-plane grain misorientation. Jc of the above samples used in XRD /-scans together with the fraction of 45 misoriented grains are listed in Table 1. Films with perfectly aligned grains have a Jc value in the range of 2–2.5 MA/cm2 except for sample D which was deposited in an earlier batch and had Jc of 3 MA/ cm2 . From the table, a good correlation was established between the film Jc value and the fraction of 45 rotated grains in the films. Jc drops significantly with the increase in the fraction of the 45 grains as indicated by the increase in the intensity of 45 peaks. Jc across individual grain boundaries in YBCO thin films has been studied as a function of misorientation angles by Dimos et al. [13] and Wen et al. [14] using MgO bicrystals. They showed that Jc drops more rapidly with increasing misorientation angle. At 45, it reaches about 1/50 of the perfect grain-aligned film. Although it has been shown that a misoriented film on a MgO substrate has a lower Jc [4,7], there is apparently no experimental data for Jc of films with a different range of the volume fraction of 45 in-plane misoriented grains. Here, we are able to present experimentally the relationship of Jc with the fraction of 45 misoriented grains for a broad range of Jc values. Jc values of the samples listed in Table 1, together with the data from several other samples, are plotted against the fraction of the 45 misoriented grains. It clearly shows that a small increase in the volume fraction of 45 misoriented grains results in a sharp drop in Jc value. Jc decreases to approximately 50% when the fraction of 45 grains increases to 0.1. Further discussion on the relationship of Jc with the volume fraction of 45 misoriented grains and an attempt to fit our data to a percolation model will be presented elsewhere [15].
2.0 1.5 1.0 0.5 0.0 0.0
0.1
0.2
0.3
0.4
Fraction of 45 deg misoriented grains Fig. 4. Experimental data of critical current density, Jc , of the YBCO films versus the fraction of 45 misoriented grains.
4500
(a)
C1s
4000
Intensity (cps)
148
3500 3000 (b)
2500 2000
(d)
(d) (a)
1500
(c) (b) (c)
1000 294
292
290
288
286
284
282
280
278
Binding Energy (eV) Fig. 5. XPS spectra of C 1s for the MgO substrates of (a) ‘‘as received’’ (sample A), (b) washed in solvents (sample J), (c) washed in Teepol detergent (sample B), and (d) IBE (300 V) cleaned (sample C).
substrates of the same batch (crystal) (a) ‘‘as received’’, (b) washed in solvents, (c) washed in detergent, and (d) IBE (VB ¼ 300 V) cleaned. Substrates for traces (a), (c), and (d) were prepared the same way as those of the films used in /-scan (corresponding to Fig. 1 traces (a), (b), and (c) respectively). The large C 1s peak at 284.6 eV was clearly shown (a) for the sample ‘‘as received’’ indicating strong carbon contamination which could have resulted from the cleavage, polishing and cleaning processes used by the manufacturer. The carbon contamination seems to be correlated
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(c)
14000
O1s
Intensity (cps)
12000
(d)
10000 (b)
8000 6000
(a)
4000 2000 0 536
534
532
530
528
526
Binding Energy (eV) Fig. 6. XPS spectra of O 1s for the MgO substrates of (a) ‘‘as received’’ (sample A), (b) washed in solvents (sample J), (c) washed in Teepol detergent (sample B), and (d) IBE (300 V) cleaned (sample C).
to the growth of 45 misoriented grains shown in Fig. 1(a). The small shoulder at the higher binding energy side of the carbon peak also indicates some by-products such as carbonates, carboxyls and possibly hydrocarbon oxides on the surface according to the relative chemical shifts of approximately 4–6 eV from the carbon peak (Ref. [16, p. 41]). Washing of MgO in standard solvents (acetone and ethanol) reduced the contamination on the surface as shown in curve (b) with the intensities of both main peak and the shoulder reduced. However, the C 1s peak is still evident after solvent washing. The washing in diluted Teepol detergent and by IBE cleaning process, in comparison, resulted in the minimum carbon peaks (traces (c) and (d)) indicating much better cleaning. However, a slight increase in the shoulders at the binding energy of 289 eV was observed for both samples indicating some degree of hydroxylation and carbonates possibly as consequence of MgO reacting with H2 O and CO2 . In the case of the IBE cleaned sample, it is possible that ion bombardment may introduce some carbonates. However, these minor level of contamination for these two samples (traces (c) and (d)) seemed to have little effect on the in-plane grain misorientation of the epitaxial YBCO films (Fig. 1(b) and (c)). In Fig. 6, the main O 1s peaks at the lower binding energy (LBE) of 530 eV are due to the substrate MgO oxygen atoms which was con-
149
firmed from the depth profile of the MgO substrate (in situ etched). The shoulders at higher binding energy (HBE) of 531–532 eV are due to oxygen in a ‘‘defective’’ chemical environment. They are likely to have resulted from hydroxides and carbonates according to the relative energy shifts of 1–2 eV ([16], p. 8). Studies of the reaction of water on MgO by several other groups [17–19] have suggested that water molecules absorb dissociatively on the MgO surface and give rise to Mg(OH)2 and OH which could be identified by O 1s core level shift to higher binding energy (2 eV, [18]). Comparing traces (a), (b), and (c) which correspond to ‘‘as received’’, solvent washed and detergent washed samples, (a) has a very weak LBE (MgO) peak and a relatively strong HBE peak (shoulder). This is due to the high degree of carbon contamination as shown in Fig. 5(a), which formed a surface carbon layer as well as by-products of hydroxides and carbonates. The strong carbon coverage for this sample is also shown by the high ratio of C/O areas in the C 1s and O 1s peaks. The relative intensities of the LBE (MgO) peaks increased for the washed samples indicating cleaner MgO surfaces. Detergent washing was found to be better than the solvent cleaning which was also shown by the relative intensities in the carbon peaks (Fig. 5). Some degree of hydroxide and carbonate contaminations is evident for the washed MgO as indicated by the small shoulders on the HBE side of the oxygen peaks. The O 1s peak is quite different for the IBE cleaned MgO (trace (d)) and the interpretation is not straightforward due to the complexity of the IBE modified surface. We believe that the slightly weak LBE (MgO) peak and higher HBE peak compared to that of the clean sample (c) is partly attributed to the effect of surface defects (oxygen and magnesium vacancies) created by ion bombardment. Consistently, the Mg 2s peak intensity of the IBE cleaned sample was also found to be weaker than that of the clean sample (c). Various studies, for example [20], have showed that the defects in MgO lattice change the interionic potentials which changes the surface energy. Such surfaces are also likely to react with the chemisorbed hydrogen and carbon monoxide to form
J. Du et al. / Physica C 400 (2004) 143–152
hydroxide and carbonate which explains the increased HBE peak at 531–532 eV observed in our IBE sample in Fig. 6(d). Other studies [17,18,21] also showed that the clean and the roughened (annealed or sputtered) MgO (1 0 0) surfaces tend to absorb water since hydroxylation occurs at atomic terrace sites and both oxygen and magnesium defective sites. Bear in mind that the IBE cleaned MgO sample was exposed to the environment for some time before XPS analysis as the IBE process was carried out in different vacuum chamber and the sample had been stored in nitrogen cabinet for 2 days before XPS measurement was undertaken. However, from the /-scan of the resulting YBCO film, the IBE (at low energy) cleaned MgO appeared to have the cleanest surface (see Fig. 1(c)). We believe that the defects caused by the IBE process are partly recovered at the YBCO film deposition temperature (700 C) in an oxygen atmosphere due to an annealing effect, and assisting the desorption of hydroxyl groups and the carbonates. Figs. 7 and 8 show XPS spectra of C 1s and O 1s of another three MgO substrates: (a) exposed in the laboratory environment for 6 weeks (sample K), (b) lithographically processed without a final IBE cleaning step (sample G), and (c) lithographically processed but with a final IBE cleaning step
2800 2600
(b)
C1s
Intensity (Cps)
2400 2200 2000 (a)
1800 1600
(c)
(a)
1400 1200
(c)
(b)
1000 800 294
292
290
288
286
284
282
280
278
Binding Energy (eV) Fig. 7. XPS spectra of C 1s for the MgO substrates of (a) exposed to the laboratory environment for 6 weeks (sample K), (b) lithographically processed without a final IBE step (sample G), and (c) lithographically processed with a final IBE cleaning step (sample I).
(c)
10000
O1s
(a)
8000
Intensity (Cps)
150
(b)
6000 4000 2000 0 536
534
532
530
528
526
Binding Energy (eV) Fig. 8. XPS spectra of O 1s for the MgO substrates of (a) exposed to the laboratory environment for 6 weeks (sample K), (b) lithographically processed without a final IBE step (sample G), and (c) lithographically processed with a final IBE cleaning step (sample I).
(sample I) ((b) and (c) corresponding to the /-scan samples used in Fig. 3(a) and (c)). Both the environment degraded MgO and the lithographically processed MgO showed an increased C 1s peaks and reduced O 1s main (LBE) peaks with increased HBE peaks (shoulders), indicating contaminations of carbon, carbonates, carboxyl and hydroxides as discussed above. In particular, the relatively strong carbon contamination for the lithographically processed MgO could originate from the PR residual. The carbonates, carboxyl and hydroxides resulted from MgO reacting with water during PR development and subsequent washing in solvents and deionised water. These contaminants are believed to contribute to the growth of the 45 rotated YBCO grains as shown in Fig. 3(a). The processed substrate after a final IBE cleaning step showed a similar spectrum (c) to the trace (d) in Figs. 5 and 6. A weak Ar peak was detected by XPS for some IBE etched samples at a higher energy (P500 eV) (not shown here). This could contribute to the observed weak 45 peaks in the /-scan spectrum in Fig. 3(b). However, in most cases, the Ar contamination due to the IBE process is very minor and the resulting 45 peaks were negligible. Nevertheless, a lower ion energy (e.g. 300 eV) and a low Ar pressure are recommended for the cleaning purpose.
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4. Discussion By correlating the XPS results with the /-scan results, it suggests that the contamination, such as carbon, carbonates and hydroxides, on the MgO surface may be responsible for the appearance of the 45 in-plane grain misorientation observed in our YBCO films. The fraction of misoriented grains depends on the degree of the contamination and determines the critical current density of the film. This work shows that preparation of MgO substrates before YBCO film deposition is crucial. The substrates ‘‘as received’’ may be contaminated due to the manufacturersÕ cutting, polishing and cleaning processes. Cleaning of substrates before film deposition is necessary. Washing in standard solvents or detergent (in ultrasonic bath) can be used to clean the substrate. Teepol industrial detergent seems to be more efficient in removing carbon contamination than solvents (Fig. 5). However, some contamination by hydroxides and carbonates were evident in the XPS measurements most likely caused by the washing. Lithographic patterning of MgO substrates contaminates the surface with carbon, carbonates and hydroxides. Exposure of MgO to the environment or storage of MgO for a long period of time, even in dry nitrogen cabinet, also degrades the surface quality (mainly introduces hydroxides and carbonates). Contamination or degradation of the MgO surface could change the relative interfacial energy of the YBCO/MgO interfaces which gives rise to the 45 rotated YBCO grains. From the film growth point of view, contamination on MgO may alter the surface structure or morphology and change the growth mechanism of the YBCO film on MgO substrate. Various researchers [1,9,23] have shown that clean or freshly cleaved MgO has the surface features of atomically flat terraces or steps with the step-height or roughness up to 1 nm. The nucleation sites of the film growth with the 0 orientation preference are formed at these steps in order to lower the surface energy [1,7,22]. Contaminated or degraded MgO surfaces do not have well defined steps as shown by our atomic force microscopy (AFM) examination and by Sum et al. [23]. The 45 orientation appears to be favoured in the absence of steps or
151
ledge sites probably due to the change of the interfacial energy of the YBCO/MgO interface. Examination of our MgO samples by AFM [24] revealed different surface morphology for various substrates. The MgO sample with strong carbon contamination (shown in Fig. 5(a)) has irregularly shaped hills of submicron size but no well-defined atomic steps or terraces with a local roughness of <0.3 nm. In other words, it has a rougher surface at a larger scale but is smoother at the atomic scale. This is because a carbon monolayer covers the MgO surface. The detergent washed MgO (shown in Fig. 5(c)) and IBE cleaned MgO surfaces have a flatter but granular or terrace appearance with a roughness of 0.7–0.9 nm and 1.0 nm respectively. In other words, the clean MgO surface is smoother at the larger scale but rougher at the atomic scale. The granular or terrace appearance of the clean MgO surface is very similar to that of the thermally annealed MgO surface obtained by Minamikawa et al. (Ref. [6], Fig. 1(b)). IBE cleaning at a low energy level did not change the surface appearance dramatically but slightly increase the roughness at the atomic level. The slightly rougher surface (up to 1 nm) seems ideal for the growth of perfectly oriented YBCO film, which is in agreement with the conclusion given by Suzuki et al. [1]. Annealing of MgO substrates at high temperatures (more than 1000 C) has been reported to be useful in removing the carbonate and hydroxide contaminants [4,6,9]. However, it is not compatible with our standard device processing techniques. IBE is a simple and standard technique used in preparing our step-edge patterns on the substrates and patterning of the YBCO devices. In this work, we have shown that a final IBE cleaning of MgO substrates prior to the YBCO film deposition is effective in removing the 45 misoriented grains in the YBCO films. It removes the degraded or contaminated MgO surface and creates a fresh and granular-looking surface suitable for the preferential growth of YBCO [1 0 0] in MgO [1 0 0] direction. A relatively low ion energy level (e.g. 300 eV) and low Ar pressure should be used to avoid Ar contamination or implantation and reduce the extent of the surface damage by ion bombardment. In our experience, a clean chamber
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and substrate platform for the final IBE are also important as any possible redeposited materials can contaminate the MgO surface and cause the growth of misoriented grains. In this work, a good correlation was established between the film Jc and the fraction of 45 misoriented grains for our YBCO films grown on MgO substrates (Fig. 4). Jc drops rapidly with an increasing amount of 45 grains due to the formation of the high-angle grain boundaries [13]. Obtaining an experimental relationship between the film Jc and the fraction of 45 misoriented grains has another significance in practice. It is quite common that film quality varies even for the same batch of films and devices due to various substrate preparation procedures and handling by different people. Optimal SQUID performance requires precise control of the junction parameters [2] which is dependent on the film Jc value once the step pattern is formed. For example, a drop of Jc by half with 10% misoriented grains in the film will result in a non-working SQUID. In the past, much time had been wasted on patterning poor quality films. We now routinely screen all films by XRD /-scan, which is quick and non-destructive, and estimate the film Jc using the above relationship prior to the device patterning. In this way, we avoid wasting time by patterning poor films and adjust the junction dimension to obtain suitable parameters for optimum device performance.
5. Conclusions The effects of MgO surface quality on the inplane grain orientation and the critical current density of epitaxial YBCO films were investigated. MgO substrates with degraded or contaminated surfaces give rise to 45 rotated grains in YBCO films and reduce the film Jc . A relationship of the film Jc with the fraction of 45 misoriented grains was experimentally established. Jc drops significantly with an increase of 45 misoriented grains. Contaminants of predominantly carbon, carboxyl, carbonates and hydroxides were identified on MgO surfaces by XPS, which resulted from the washing procedures, exposure to environment and lithographic processing. IBE cleaning at a low
energy level prior to film deposition was found to be very effective in eliminating the in-plane grain misorientations in YBCO films.
References [1] H. Suzuki, Y. Fujiwara, Y. Hirotsu, T. Yamashita, T. Oikawa, Jpn. J. Appl. Phys. 32 (1993) 1601. [2] J. Du, S.K.H. Lam, D.L. Tilbrook, C.P. Foley, Supercond. Sci. Technol. 15 (2002) 165. [3] J. Du, IEEE Trans. Appl. Supercond. 13 (2003) 865. [4] B.H. Moeckly, S.E. Russek, D.K. Lathrop, R.A. Buhrman, J. Li, J.W. Mayer, Appl. Phys. Lett. 57 (1990) 1687. [5] S. McKernan, M.G. Norton, C.B. Carter, J. Mater. Res. 7 (1992) 1052. [6] T. Minamikawa, T. Suzuki, Y. Yonezawa, K. Segawa, A. Morimoto, T. Shimizu, Jpn. J. Appl. Phys. 34 (1995) 4038. [7] T. Kitamura, J.G. Wen, Y. Shiohara, N. Koshizuka, I. Hirabayashi, S. Tanaka, Y. Sugawara, Y. Ikuhara, Physica C 262 (1996) 120. [8] K. Nomura, S. Hoshi, X. Yao, K. Kakimoto, Y. Nakamura, T. Izumi, Y. Shiohara, J. Mater. Res. 16 (2001) 979. [9] M.G. Norton, S.R. Summertelt, C.B. Carter, Appl. Phys. Lett. 56 (1990) 2246. [10] E.E. Mitchell, S. Gnanarajan, K.L. Green, C.P. Foley, Thin Solid Films 437 (2003) 101. [11] C.P. Foley, E.E. Mitchell, S.K.H. Lam, B. Sankrithyan, Y.M. Wilson, D.L. Tilbrook, S.J. Morris, IEEE Trans. Appl. Supercond. 9 (1999) 4281. [12] B. Utz, R. Semerad, M. Bauer, W. Prusseit, P. Berberich, H. Kinder, IEEE Trans. Appl. Supercond. 7 (1997) 1272. [13] D. Dimos, P. Chaudari, J. Mannhart, Phys. Rev. B 41 (1990) 4038. [14] J.G. Wen, T. Takagi, N. Koshizuka, Supercond. Sci. Technol. 13 (2000) 820. [15] S. Gnanarajan, J. Du, to be published. [16] J. Chastain, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corporation, Physical Electronics Division, USA, 1992. [17] H. Onishi, C. Egawa, T. Aruga, Y. Iwasawa, Surf. Sci. 191 (1987) 479. [18] D. Abriou, J. Jupille, Surf. Sci. 430 (1999) L527. [19] P. Liu, T. Kendelewicz, G.E. Brown Jr., G.A. Parks, Surf. Sci. 412/413 (1998) 287. [20] E.A. Colbourn, Surf. Sci. Rep. 15 (1992) 281. [21] P. Liu, T. Kendelewicz, G.E. Brown Jr., Surf. Sci. 412/413 (1998) 315. [22] T.S. Ravi, D.M. Hwang, R. Ramesh, S.W. Chan, L. Nazar, C.Y. Chen, A. Inam, T. Venkatesan, Phys. Rev. B 42 (1990) 10141. [23] R. Sum, H.P. Lang, H.J. G€ untherodt, Physica C 242 (1995) 174. [24] J. Du, J. Hermann, unpublished results.
Influence of MgO surface conditions on the in-plane crystal orientation and critical current density of epitaxial YBCO films J. Du *, S. Gnanarajan, A. Bendavid CSIRO Telecommunications and Industrial Physics, P.O. Box 218, Lindfield, NSW 2070, Australia Received 7 July 2003; accepted 24 July 2003
Abstract The effect of magnesium oxide (MgO) surface conditions on in-plane grain orientation and critical current density of epitaxial YBa2 Cu3 O7 (YBCO) films was systematically investigated. The MgO substrates were either ‘‘as received’’ or stored for some time, cleaned using different methods and lithographically prepared for our step-edge junction devices. The YBCO films were grown via reactive thermal co-evaporation by Theva, GmbH. The surface characterisation of MgO substrates was studied using X-ray photoelectron spectroscopy (XPS). The in-plane grain orientation of the YBCO films was studied by means of X-ray diffraction (XRD) /-scan and the critical current density was measured for the XRD scanned samples. The surface condition of the MgO substrates was found to have a strong influence on the inplane grain orientation and the critical current density of the YBCO films. The MgO substrates with a degraded or contaminated surface gave rise to 45 grain misorientation in YBCO films and reduced the critical current density. A final process step using a low energy Ar ion beam etching (IBE) of the MgO substrates prior to the YBCO film deposition was found effective in removing the in-plane grain misorientation and promoting the growth of perfectly aligned c-axis YBCO films. 2003 Elsevier B.V. All rights reserved. PACS: 74.76.Bz; 81.65.Cf Keywords: YBa2 Cu3 O7 ; Magnesium oxide (MgO); Grain orientation; Ion beam etch; XRD phi-scan; X-ray photoelectron spectroscopy
1. Introduction Single-crystal magnesium oxide (MgO) is one of the popular substrates for the growth of high
*
Corresponding author. Tel.: +61-2-9413-7107; fax: +61-29413-7161. E-mail address: [email protected] (J. Du).
quality YBa2 Cu3 Ox (YBCO) thin films due to its low dielectric loss, thermal expansion coefficient similar to that of YBCO, and low cost despite the large lattice mismatch with YBCO of 9% [1]. YBCO (1 0 0) films were grown on MgO (1 0 0) substrates for our step-edge junction superconducting quantum interference devices (SQUIDs) and other junction based devices. Good control of the Josephson junction parameters requires a
0921-4534/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2003.07.005
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reproducible and high quality growth of YBCO films. Deviation from the required specification of the film properties usually results in either nonworking devices or a need for post-trimming the devices [2,3]. Step-edge patterns were routinely prepared on MgO substrates using a standard lithographic process and ion milling technique prior to the deposition of the YBCO thin films. This lithographic processing of a MgO substrate was found to have a detrimental effect on the surface quality and, consequently, the quality of the epitaxial YBCO film. Similarly, exposure of MgO substrates to the environment or even being stored in dry nitrogen for a period of time also has an adverse effect on the quality of the YBCO films. The deterioration of MgO substrates by exposure to the environment or during processing is a major disadvantage for YBCO thin film growth. In-plane grain misorientation has been observed in YBCO films grown on MgO (1 0 0) substrates by other groups [1,4–8]. Grain misorientation varied in angle [1,4,7], and could be affected by MgO surface polishing techniques [4,5] and by YBCO film deposition techniques and conditions [1,6,8]. Annealing of MgO substrates at high temperatures prior to film deposition has been found to be useful in improving in-plane grain orientation in the YBCO films [4,6,9]. In our YBCO (1 0 0)/MgO (1 0 0) films, the grains were found to be either perfectly oriented or 45 misoriented. The fraction of the 45 misoriented grains in the film was found to be strongly associated with the surface condition of the MgO substrate due to the preparation method. The existence of 45 misoriented grains caused the film Jc to drop significantly and reduced our device yield. Although, there is some discussion in the literature on the growth mechanism of misoriented grains and the dependency of film Jc on the misorientation angle, there is, however, a lack of a systematic study on the relationship of the grain misorientation in YBCO films and the MgO surface quality as result of different substrate preparation methods. We found that various degrees of 45 in-plane misorientation could be resulted in the same batch of YBCO films grown on the same batch of MgO substrates due to different substrate cleaning and processing methods. There is also a lack of experimental data in the literature
showing the relationship of film Jc with the fraction of 45 misoriented grains for a broad range of Jc values. In order to control the quality of YBCO films and the yield of devices, we need to understand the cause of the 45 grain misorientation observed in our YBCO/MgO (1 0 0) films and its effect on the critical current density. Furthermore, we need to develop a cleaning process to improve the MgO surface quality after being lithographically processed or stored for a period of time prior to the film deposition. In this work, we systematically investigated the influence of various surface conditions of MgO on the in-plane grain orientation and the critical current density of the YBCO films. The different surface conditions of MgO substrates considered were due to various preparation methods. We describe a new process to clean the MgO surface to ensure the growth of perfectly grain-aligned YBCO films.
2. Experimental Single-crystal MgO (1 0 0) substrates of 10 mm · 10 mm · 0.5 mm size were supplied by ESCETE, Holland, and Crystal GmbH, Germany. In our experience, commercially available MgO substrates varied in quality from supplier to supplier and from batch to batch in terms of surface defects, roughness [10] and contaminants. We usually screen out the substrates with excessive polish/scratch lines and other visual defects by using an optical microscope at ·1000 magnification. To reduce possible variables in the substrates and the films, we compared the substrates from the same batch of the same supplier and prepared with our standard washing and processing methods. MgO samples were prepared in different ways: ‘‘as received’’ without treatment, washed in detergent or solvents, exposed to the laboratory environment or stored in a dry nitrogen environment for a period of time, and lithographically prepared with or without a final Ar ion beam etching (IBE), as summarised in Table 1. In the standard washing process, the sample was either washed in Teepol detergent, diluted with deionised (DI) water in 4:1 ratio, in an ultrasonic bath for 2 min, followed
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Table 1 Information about the MgO substrates and properties of the resulting YBCO films Sample ID
MgO substrate treatment
Fraction of 45 grains in YBCO
YBCO film Jc (MA/cm2 )
Aa Ba
‘‘As received’’ without any treatment Washed in diluted Teepol detergent in ultrasonic bath, followed by DI water rinse and N2 blow dry IBE cleaned for 8 min at VB ¼ 300 V Fresh MgO After being stored in dry N2 cabinet for 20 months IBE (VB ¼ 300 V) cleaned after being stored for 20 months in dry N2 Lithographically patterned without a final IBE Lithographically patterned with a final IBE at VB ¼ 500 V Lithographically patterned with a final IBE at VB ¼ 300 V Washed in acetone in ultrasonic bath, followed by ethanol rinse and N2 blow dry Exposed to laboratory environment for 6 weeks
0.30 0.01
0.005 2.0
Ca Db Eb Fb Gc Hc Ic Ja Kc
0.00 0.00 0.20 0.00 0.11 0.02 0.00
2.4 3.0 0.22 2.4 1.1 1.9 2.2
a
MgO substrates of a same batch from Crystal. MgO substrates of a same batch from ESCETE. c MgO substrates of a same batch from Crystal. b
by a deionised water rinse for less than 1 min and blown dry with nitrogen gas, or washed in acetone in an ultrasonic bath, followed by ethanol rinse and blown dry with nitrogen gas. Teepol detergent is a secondary alcohol sulphate and is regularly used to clean various substrates. The lithographically processed MgO substrates were prepared using our standard processing steps for step-edge patterns [11] which include photoresist (PR) spuncoating, UV exposure, PR development, the IBE etching at a non-normal angle, washing in acetone and ethanol, and nitrogen blow dry. The final IBE cleaning process involves refreshing the MgO substrate by etching away a thin surface layer using Ar ion IBE at a relatively low energy (e.g. 300 eV) and at an Ar pressure of 63 · 104 mbar, prior to the YBCO film deposition. YBCO films of 200–250 nm thicknesses were deposited on the MgO substrates by THEVA GmbH, Germany using a reactive thermal coevaporation technique [12]. All samples except for one used in this investigation were deposited in two batches. Therefore, possible variations due to film deposition procedures were minimised. There is no observable misorientation in the c-axis for these films as determined by XRD h–2h measurements and grain misorientation only occurred in the ab plane. The critical current density, Jc ,
of the perfectly grain-oriented films was in the range of 2–3 MA/cm2 which was desired for our SQUIDs. In-plane growth orientation of the YBCO films was investigated by X-ray diffraction (XRD) /scans using a Philips XÕPert diffraction system and with the Cu-Ka radiation operated at 45 kV and 40 mA. The scan was performed on the YBCO (1 0 3) plane which has a 45 angle orientation to the substrate plane. The volume fraction of the misoriented grains was determined from the relative peak intensities. The chemical composition of the surface of the MgO substrates was assessed by X-ray photoelectron spectroscopy (XPS) using a Specs (SAGE 150) photoelectron spectrometer operated with an Mg X-ray source. The binding energies of the photoelectron peaks from the films were referenced to the C 1s peak at 284.6 eV to compensate for any surface charging. The critical current density, Jc , of YBCO films at 77 K was determined either using an inductive Jc measurement by THEVA or an in-house standard 4-point transport current–voltage (I–V ) measurement. In the transport current method, microbridges of width 50 or 100 lm were patterned and the critical current, Ic , of the microbridges was measured. Ic was determined from the onset of a nonzero voltage of I–V curve using a 1 lV criterion.
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Film Jc was given by Ic =A, where A is the crosssectional area of the microbridge.
3. Results 3.1. In-plane orientation of YBCO films
counts/s
60k 50k 40k 30k 20k 10k 0
(b) sample B (detergent washed)
counts/s
70k 60k 50k 40k 30k 20k 10k 0 70k 60k 50k 40k 30k 20k 10k 0
(c) sample C (IBE cleaned)
counts/s
XRD /-scan analysis has been undertaken on a large number of YBCO films. The results shown here are typical for those of similar substrate conditions. Information about the MgO substrates used in this paper and the /-scan results of the YBCO films grown on these substrates are summarised in Table 1. Fig. 1 compares the /-scan spectra of the YBCO films on the MgO substrates of the same batch (Crystal): (a) ‘‘as received’’ without washing (sample A), (b) washed in deter-
(a) sample A ( "as received")
0
50
100
150
200
250
300
350
phi (deg) Fig. 1. XRD /-scan spectra for the YBCO films grown on the MgO substrates of (a) ‘‘as received’’ (sample A), (b) washed in detergent (sample B), and (c) IBE (300 V) cleaned (sample C).
gent (sample B) and (c) IBE (300 V) cleaned (sample C). The peaks at 0, 90, 180 and 360 come from the perfectly aligned grains (we denote them as 0 rotation). The fourfold symmetry reflects the cubic structures and the similar values of the a- and b-axes. The 45 peaks represent a 45 rotation of some YBCO grains in relationship to the substrate, i.e. the a-axis of the film [1 0 0] is parallel to the [1 1 0] direction rather than [1 0 0] direction of the substrate. Strong 45 misorientation peaks are clearly shown for the film grown on the ‘‘as received’’ substrate (trace (a)). Only slight 45 peaks were visible for the washed substrate (trace (b)) suggesting a substantial improvement in in-plane crystal orientation of the epitaxial film. IBE cleaning of the substrate completely eliminated the misorientation peaks (trace (c)) indicating a perfectly grain-aligned YBCO film. Storage of MgO substrates was found to affect the surface quality and therefore the quality of epitaxial YBCO films. Fig. 2 shows the /-scan spectra of YBCO films on another batch MgO substrates (ESCETE); (a) ‘‘fresh’’ substrate (sample D), (b) after being stored in dry nitrogen cabinet for 20 months (sample E) with some short exposures to the environment when taking out other substrates, and (c) IBE etched after being stored for 20 months (sample F). Films on substrates E and F were deposited together and the film on substrate D was deposited 20 months earlier. A perfectly aligned YBCO film was obtained from the fresh substrate (trace (a)), but 45 peaks were observed in the film grown on the 20month old MgO (trace (b)). This shows that storage of MgO has some influence on YBCO film quality due to degradation of the MgO surface. However, our experiment has shown that the degradation of MgO is rather slow in a dry nitrogen environment compared to that exposed in the laboratory environment. The film grown on the IBE etched old substrate (trace (c)) showed no misorientation peaks suggesting a perfectly aligned YBCO film. The implication is that IBE etching can be used to refresh the degraded MgO substrate due to storage. Fig. 3 compares the /-scan results on the YBCO films grown on (a) patterned MgO substrate without a final IBE etching process (sample G),
J. Du et al. / Physica C 400 (2004) 143–152
counts
70k 60k 50k 40k 30k 20k 10k 0
counts
80k 70k 60k 50k 40k 30k 20k 10k 0
counts
(a) sample D ("fresh")
100k
80k 70k 60k 50k 40k 30k 20k 10k 0
counts
80k 60k 40k 20k
counts
0 80k 70k 60k 50k 40k 30k 20k 10k 0
(b) sample E (20 mons old)
100k
counts
(a) sample G (patterned, no final IBE)
(b) sample H (patterned, IBE at 500V)
(c) sample F (20 mons old, IBE)
(c) sample I (patterned, IBE at 300V)
80k 60k 40k 20k 0 0
50
100
150
200
250
300
350
phi (deg) Fig. 2. XRD /-scan spectra for the YBCO films grown on the MgO substrates of (a) fresh (sample D), (b) 20-month old (sample E), and (c) 20-month old but IBE etched (sample F). These three substrates were from the same batch. The films of (b) and (c) were deposited together.
(b) patterned MgO substrate with a final IBE etching at 500 V for 8 min (sample H), and (c) with a final IBE etching process at 300 V for 8 min (sample I). The three samples went through the same lithographic processes before the final IBE cleaning and the films were deposited together. The results showed that the lithographic process degrades the MgO surface and results in the growth of 45 misoriented YBCO grains. A final IBE process was shown to be effective in removing the misoriented grains as shown in (b) and (c). However, IBE at higher energies (e.g. 500 eV) can affect the in-plane grain orientation slightly as indicated by the weak 45 peaks in Fig. 3(b). It could be due to minor Ar contamination or implantation on the surface as probed by XPS or possible surface defects caused by ion bombardment. In most cases, the misorientation peaks for the ion-etched
147
0
50
100
150 200 phi (deg)
250
300
350
Fig. 3. XRD /-scan spectra for the YBCO films grown on the MgO substrates of (a) patterned without a final IBE (sample G), (b) patterned with a final IBE at 500 V for 8 min (sample H), and (c) with a final IBE at 300 V for 8 min (sample I).
samples were negligible. A perfectly grain-oriented film was obtained from the low energy (300 eV) IBE cleaned MgO substrate indicated by 0 only peaks in trace (c). Results here suggest that the final IBE process of the patterned MgO substrates at a low energy level prior to the film deposition can be used to remove the contaminated or degenerated surface layer due to lithographic processing. The final IBE cleaning of the patterned MgO substrates prior to YBCO film deposition has become a standard procedure for fabricating our step-edge junction devices. 3.2. Critical current density Jc Critical current density, Jc , of the YBCO films grown on the MgO substrates was assessed for the
J. Du et al. / Physica C 400 (2004) 143–152
3.3. Surface characterisation of MgO by XPS XPS was used to investigate the surface of the MgO substrates that have been prepared or cleaned in various methods. The major differences in the XPS spectra were found in the carbon (C 1s) and oxygen (O 1s) peaks. Figs. 5 and 6 compare the XPS spectra of C 1s and O 1s for four MgO
2.5
YBCO film J c (MA/cm )2
films with different fractions of 45 in-plane grain misorientation. Jc of the above samples used in XRD /-scans together with the fraction of 45 misoriented grains are listed in Table 1. Films with perfectly aligned grains have a Jc value in the range of 2–2.5 MA/cm2 except for sample D which was deposited in an earlier batch and had Jc of 3 MA/ cm2 . From the table, a good correlation was established between the film Jc value and the fraction of 45 rotated grains in the films. Jc drops significantly with the increase in the fraction of the 45 grains as indicated by the increase in the intensity of 45 peaks. Jc across individual grain boundaries in YBCO thin films has been studied as a function of misorientation angles by Dimos et al. [13] and Wen et al. [14] using MgO bicrystals. They showed that Jc drops more rapidly with increasing misorientation angle. At 45, it reaches about 1/50 of the perfect grain-aligned film. Although it has been shown that a misoriented film on a MgO substrate has a lower Jc [4,7], there is apparently no experimental data for Jc of films with a different range of the volume fraction of 45 in-plane misoriented grains. Here, we are able to present experimentally the relationship of Jc with the fraction of 45 misoriented grains for a broad range of Jc values. Jc values of the samples listed in Table 1, together with the data from several other samples, are plotted against the fraction of the 45 misoriented grains. It clearly shows that a small increase in the volume fraction of 45 misoriented grains results in a sharp drop in Jc value. Jc decreases to approximately 50% when the fraction of 45 grains increases to 0.1. Further discussion on the relationship of Jc with the volume fraction of 45 misoriented grains and an attempt to fit our data to a percolation model will be presented elsewhere [15].
2.0 1.5 1.0 0.5 0.0 0.0
0.1
0.2
0.3
0.4
Fraction of 45 deg misoriented grains Fig. 4. Experimental data of critical current density, Jc , of the YBCO films versus the fraction of 45 misoriented grains.
4500
(a)
C1s
4000
Intensity (cps)
148
3500 3000 (b)
2500 2000
(d)
(d) (a)
1500
(c) (b) (c)
1000 294
292
290
288
286
284
282
280
278
Binding Energy (eV) Fig. 5. XPS spectra of C 1s for the MgO substrates of (a) ‘‘as received’’ (sample A), (b) washed in solvents (sample J), (c) washed in Teepol detergent (sample B), and (d) IBE (300 V) cleaned (sample C).
substrates of the same batch (crystal) (a) ‘‘as received’’, (b) washed in solvents, (c) washed in detergent, and (d) IBE (VB ¼ 300 V) cleaned. Substrates for traces (a), (c), and (d) were prepared the same way as those of the films used in /-scan (corresponding to Fig. 1 traces (a), (b), and (c) respectively). The large C 1s peak at 284.6 eV was clearly shown (a) for the sample ‘‘as received’’ indicating strong carbon contamination which could have resulted from the cleavage, polishing and cleaning processes used by the manufacturer. The carbon contamination seems to be correlated
J. Du et al. / Physica C 400 (2004) 143–152
(c)
14000
O1s
Intensity (cps)
12000
(d)
10000 (b)
8000 6000
(a)
4000 2000 0 536
534
532
530
528
526
Binding Energy (eV) Fig. 6. XPS spectra of O 1s for the MgO substrates of (a) ‘‘as received’’ (sample A), (b) washed in solvents (sample J), (c) washed in Teepol detergent (sample B), and (d) IBE (300 V) cleaned (sample C).
to the growth of 45 misoriented grains shown in Fig. 1(a). The small shoulder at the higher binding energy side of the carbon peak also indicates some by-products such as carbonates, carboxyls and possibly hydrocarbon oxides on the surface according to the relative chemical shifts of approximately 4–6 eV from the carbon peak (Ref. [16, p. 41]). Washing of MgO in standard solvents (acetone and ethanol) reduced the contamination on the surface as shown in curve (b) with the intensities of both main peak and the shoulder reduced. However, the C 1s peak is still evident after solvent washing. The washing in diluted Teepol detergent and by IBE cleaning process, in comparison, resulted in the minimum carbon peaks (traces (c) and (d)) indicating much better cleaning. However, a slight increase in the shoulders at the binding energy of 289 eV was observed for both samples indicating some degree of hydroxylation and carbonates possibly as consequence of MgO reacting with H2 O and CO2 . In the case of the IBE cleaned sample, it is possible that ion bombardment may introduce some carbonates. However, these minor level of contamination for these two samples (traces (c) and (d)) seemed to have little effect on the in-plane grain misorientation of the epitaxial YBCO films (Fig. 1(b) and (c)). In Fig. 6, the main O 1s peaks at the lower binding energy (LBE) of 530 eV are due to the substrate MgO oxygen atoms which was con-
149
firmed from the depth profile of the MgO substrate (in situ etched). The shoulders at higher binding energy (HBE) of 531–532 eV are due to oxygen in a ‘‘defective’’ chemical environment. They are likely to have resulted from hydroxides and carbonates according to the relative energy shifts of 1–2 eV ([16], p. 8). Studies of the reaction of water on MgO by several other groups [17–19] have suggested that water molecules absorb dissociatively on the MgO surface and give rise to Mg(OH)2 and OH which could be identified by O 1s core level shift to higher binding energy (2 eV, [18]). Comparing traces (a), (b), and (c) which correspond to ‘‘as received’’, solvent washed and detergent washed samples, (a) has a very weak LBE (MgO) peak and a relatively strong HBE peak (shoulder). This is due to the high degree of carbon contamination as shown in Fig. 5(a), which formed a surface carbon layer as well as by-products of hydroxides and carbonates. The strong carbon coverage for this sample is also shown by the high ratio of C/O areas in the C 1s and O 1s peaks. The relative intensities of the LBE (MgO) peaks increased for the washed samples indicating cleaner MgO surfaces. Detergent washing was found to be better than the solvent cleaning which was also shown by the relative intensities in the carbon peaks (Fig. 5). Some degree of hydroxide and carbonate contaminations is evident for the washed MgO as indicated by the small shoulders on the HBE side of the oxygen peaks. The O 1s peak is quite different for the IBE cleaned MgO (trace (d)) and the interpretation is not straightforward due to the complexity of the IBE modified surface. We believe that the slightly weak LBE (MgO) peak and higher HBE peak compared to that of the clean sample (c) is partly attributed to the effect of surface defects (oxygen and magnesium vacancies) created by ion bombardment. Consistently, the Mg 2s peak intensity of the IBE cleaned sample was also found to be weaker than that of the clean sample (c). Various studies, for example [20], have showed that the defects in MgO lattice change the interionic potentials which changes the surface energy. Such surfaces are also likely to react with the chemisorbed hydrogen and carbon monoxide to form
J. Du et al. / Physica C 400 (2004) 143–152
hydroxide and carbonate which explains the increased HBE peak at 531–532 eV observed in our IBE sample in Fig. 6(d). Other studies [17,18,21] also showed that the clean and the roughened (annealed or sputtered) MgO (1 0 0) surfaces tend to absorb water since hydroxylation occurs at atomic terrace sites and both oxygen and magnesium defective sites. Bear in mind that the IBE cleaned MgO sample was exposed to the environment for some time before XPS analysis as the IBE process was carried out in different vacuum chamber and the sample had been stored in nitrogen cabinet for 2 days before XPS measurement was undertaken. However, from the /-scan of the resulting YBCO film, the IBE (at low energy) cleaned MgO appeared to have the cleanest surface (see Fig. 1(c)). We believe that the defects caused by the IBE process are partly recovered at the YBCO film deposition temperature (700 C) in an oxygen atmosphere due to an annealing effect, and assisting the desorption of hydroxyl groups and the carbonates. Figs. 7 and 8 show XPS spectra of C 1s and O 1s of another three MgO substrates: (a) exposed in the laboratory environment for 6 weeks (sample K), (b) lithographically processed without a final IBE cleaning step (sample G), and (c) lithographically processed but with a final IBE cleaning step
2800 2600
(b)
C1s
Intensity (Cps)
2400 2200 2000 (a)
1800 1600
(c)
(a)
1400 1200
(c)
(b)
1000 800 294
292
290
288
286
284
282
280
278
Binding Energy (eV) Fig. 7. XPS spectra of C 1s for the MgO substrates of (a) exposed to the laboratory environment for 6 weeks (sample K), (b) lithographically processed without a final IBE step (sample G), and (c) lithographically processed with a final IBE cleaning step (sample I).
(c)
10000
O1s
(a)
8000
Intensity (Cps)
150
(b)
6000 4000 2000 0 536
534
532
530
528
526
Binding Energy (eV) Fig. 8. XPS spectra of O 1s for the MgO substrates of (a) exposed to the laboratory environment for 6 weeks (sample K), (b) lithographically processed without a final IBE step (sample G), and (c) lithographically processed with a final IBE cleaning step (sample I).
(sample I) ((b) and (c) corresponding to the /-scan samples used in Fig. 3(a) and (c)). Both the environment degraded MgO and the lithographically processed MgO showed an increased C 1s peaks and reduced O 1s main (LBE) peaks with increased HBE peaks (shoulders), indicating contaminations of carbon, carbonates, carboxyl and hydroxides as discussed above. In particular, the relatively strong carbon contamination for the lithographically processed MgO could originate from the PR residual. The carbonates, carboxyl and hydroxides resulted from MgO reacting with water during PR development and subsequent washing in solvents and deionised water. These contaminants are believed to contribute to the growth of the 45 rotated YBCO grains as shown in Fig. 3(a). The processed substrate after a final IBE cleaning step showed a similar spectrum (c) to the trace (d) in Figs. 5 and 6. A weak Ar peak was detected by XPS for some IBE etched samples at a higher energy (P500 eV) (not shown here). This could contribute to the observed weak 45 peaks in the /-scan spectrum in Fig. 3(b). However, in most cases, the Ar contamination due to the IBE process is very minor and the resulting 45 peaks were negligible. Nevertheless, a lower ion energy (e.g. 300 eV) and a low Ar pressure are recommended for the cleaning purpose.
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4. Discussion By correlating the XPS results with the /-scan results, it suggests that the contamination, such as carbon, carbonates and hydroxides, on the MgO surface may be responsible for the appearance of the 45 in-plane grain misorientation observed in our YBCO films. The fraction of misoriented grains depends on the degree of the contamination and determines the critical current density of the film. This work shows that preparation of MgO substrates before YBCO film deposition is crucial. The substrates ‘‘as received’’ may be contaminated due to the manufacturersÕ cutting, polishing and cleaning processes. Cleaning of substrates before film deposition is necessary. Washing in standard solvents or detergent (in ultrasonic bath) can be used to clean the substrate. Teepol industrial detergent seems to be more efficient in removing carbon contamination than solvents (Fig. 5). However, some contamination by hydroxides and carbonates were evident in the XPS measurements most likely caused by the washing. Lithographic patterning of MgO substrates contaminates the surface with carbon, carbonates and hydroxides. Exposure of MgO to the environment or storage of MgO for a long period of time, even in dry nitrogen cabinet, also degrades the surface quality (mainly introduces hydroxides and carbonates). Contamination or degradation of the MgO surface could change the relative interfacial energy of the YBCO/MgO interfaces which gives rise to the 45 rotated YBCO grains. From the film growth point of view, contamination on MgO may alter the surface structure or morphology and change the growth mechanism of the YBCO film on MgO substrate. Various researchers [1,9,23] have shown that clean or freshly cleaved MgO has the surface features of atomically flat terraces or steps with the step-height or roughness up to 1 nm. The nucleation sites of the film growth with the 0 orientation preference are formed at these steps in order to lower the surface energy [1,7,22]. Contaminated or degraded MgO surfaces do not have well defined steps as shown by our atomic force microscopy (AFM) examination and by Sum et al. [23]. The 45 orientation appears to be favoured in the absence of steps or
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ledge sites probably due to the change of the interfacial energy of the YBCO/MgO interface. Examination of our MgO samples by AFM [24] revealed different surface morphology for various substrates. The MgO sample with strong carbon contamination (shown in Fig. 5(a)) has irregularly shaped hills of submicron size but no well-defined atomic steps or terraces with a local roughness of <0.3 nm. In other words, it has a rougher surface at a larger scale but is smoother at the atomic scale. This is because a carbon monolayer covers the MgO surface. The detergent washed MgO (shown in Fig. 5(c)) and IBE cleaned MgO surfaces have a flatter but granular or terrace appearance with a roughness of 0.7–0.9 nm and 1.0 nm respectively. In other words, the clean MgO surface is smoother at the larger scale but rougher at the atomic scale. The granular or terrace appearance of the clean MgO surface is very similar to that of the thermally annealed MgO surface obtained by Minamikawa et al. (Ref. [6], Fig. 1(b)). IBE cleaning at a low energy level did not change the surface appearance dramatically but slightly increase the roughness at the atomic level. The slightly rougher surface (up to 1 nm) seems ideal for the growth of perfectly oriented YBCO film, which is in agreement with the conclusion given by Suzuki et al. [1]. Annealing of MgO substrates at high temperatures (more than 1000 C) has been reported to be useful in removing the carbonate and hydroxide contaminants [4,6,9]. However, it is not compatible with our standard device processing techniques. IBE is a simple and standard technique used in preparing our step-edge patterns on the substrates and patterning of the YBCO devices. In this work, we have shown that a final IBE cleaning of MgO substrates prior to the YBCO film deposition is effective in removing the 45 misoriented grains in the YBCO films. It removes the degraded or contaminated MgO surface and creates a fresh and granular-looking surface suitable for the preferential growth of YBCO [1 0 0] in MgO [1 0 0] direction. A relatively low ion energy level (e.g. 300 eV) and low Ar pressure should be used to avoid Ar contamination or implantation and reduce the extent of the surface damage by ion bombardment. In our experience, a clean chamber
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and substrate platform for the final IBE are also important as any possible redeposited materials can contaminate the MgO surface and cause the growth of misoriented grains. In this work, a good correlation was established between the film Jc and the fraction of 45 misoriented grains for our YBCO films grown on MgO substrates (Fig. 4). Jc drops rapidly with an increasing amount of 45 grains due to the formation of the high-angle grain boundaries [13]. Obtaining an experimental relationship between the film Jc and the fraction of 45 misoriented grains has another significance in practice. It is quite common that film quality varies even for the same batch of films and devices due to various substrate preparation procedures and handling by different people. Optimal SQUID performance requires precise control of the junction parameters [2] which is dependent on the film Jc value once the step pattern is formed. For example, a drop of Jc by half with 10% misoriented grains in the film will result in a non-working SQUID. In the past, much time had been wasted on patterning poor quality films. We now routinely screen all films by XRD /-scan, which is quick and non-destructive, and estimate the film Jc using the above relationship prior to the device patterning. In this way, we avoid wasting time by patterning poor films and adjust the junction dimension to obtain suitable parameters for optimum device performance.
5. Conclusions The effects of MgO surface quality on the inplane grain orientation and the critical current density of epitaxial YBCO films were investigated. MgO substrates with degraded or contaminated surfaces give rise to 45 rotated grains in YBCO films and reduce the film Jc . A relationship of the film Jc with the fraction of 45 misoriented grains was experimentally established. Jc drops significantly with an increase of 45 misoriented grains. Contaminants of predominantly carbon, carboxyl, carbonates and hydroxides were identified on MgO surfaces by XPS, which resulted from the washing procedures, exposure to environment and lithographic processing. IBE cleaning at a low
energy level prior to film deposition was found to be very effective in eliminating the in-plane grain misorientations in YBCO films.
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