Degradation of MgB 2 Ultrathin Films Under Different Environmental Conditions

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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI10.1109/TASC.2014.2379722, IEEE Transactions on Applied Superconductivity

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. , NO. , NOVEMBER 2014 1

Degradation of MgB2 Ultrathin Films UnderDifferent Environmental Conditions

Da Wang, Chen Zhang, Jun Zhang, Yan Zhang, Qing-Rong Feng, Yue Wang, and Zi-Zhao Gan

Abstract—MgB2 ultrathin films have recently been recognizedto have potential for making sensitive superconducting devicesworking at relatively high temperatures. Investigation on thestability of MgB2 ultrathin films under various environmentalconditions should be helpful in evaluating their prospect inpractical device applications. Here we present a comparativestudy on the degradation of MgB2 films of thicknesses of∼ 15 nm grown by hybrid physical-chemical vapor deposition(HPCVD) method and then placed at room temperature underthree different kinds of environments: in air, in a desiccatorwith reduced humidity, and in a vacuum chamber with furtherreduced atmospheric pressure. The study was performed bymeasuring the normal-state resistance and the superconductingproperties such as the superconducting critical temperature Tc

and the upper critical field Hc2 of the films after differentstorage times, in conjunction with supplemental scanning electronmicroscopy (SEM) and X-ray diffraction (XRD) measurements. Ithas been found that, compared with the film exposed to air whichstarted to show apparent degradation when the storage time wasbeyond about one month, the films kept in a desiccator or lowvacuum exhibited less degradation. In particular, the film in lowvacuum remained stable throughout the measured time periodof ∼ 4 months. The results suggest that the degradation of MgB2

ultrathin films in ambient environment may mainly arise frominteraction of the sample with the moisture in the air and that,in a dry air or preferably vacuum or an N2 environment, MgB2

ultrathin films can maintain the reasonable long-time stabilityfor superconducting applications.

Index Terms—Magnesium compounds, thin films, degradation.

I. INTRODUCTION

S INCE the discovery of superconductivity at 39 K in MgB2

[1], much attention has been paid to exploring its basicphysical properties as well as its suitability and potentialfor superconducting applications. As the first experimentally-confirmed two-band superconductor, MgB2 exhibits manyintriguing features in its physical properties [2]; MgB2 isalso promising in superconducting applications owing to itsappealing characters such as the high superconducting criticaltemperature Tc beyond the reach of low-Tc materials and itslack of weakly-linked grain boundaries in contrast to high-Tc cuprate superconductors. Recently, ultrathin MgB2 filmsin thicknesses around or below 20 nm have attracted manyinterests from the community because of their prospects in de-veloping sensitive superconducting devices, such as supercon-

This work was supported in part by the Ministry of Science and Technologyof China under 973 Program 2011CBA00106, by the National Natural ScienceFoundation of China under Grant 11074008, and by the Research Fund forthe Doctoral Program of Higher Education under Grant 20100001120006.

The authors are with the Applied Superconductivity Center, State KeyLaboratory for Mesoscopic Physics, School of Physics, Peking University,Beijing 100871, China (e-mail: [email protected]).

Manuscript received November 19, 2014.

ducting single photon detectors (SSPDs) [3] and hot electronbolometers (HEBs) [4]. Currently, these kinds of devices aremade mostly from ultrathin films of low-Tc superconductorslike NbN with operating temperatures at ∼ 4 K. Using MgB2,it is expected that the operating temperature of the device canbe raised to ∼ 20 K, which would considerably reduce thecryogenic cost. Moreover, it is suggested that by using MgB2

the performance of the device, such as the optical responsetime of SSPDs and the gain bandwidth of HEB mixers, couldbe improved as well. Under this circumstance, several groupshave reported the growth of MgB2 ultrathin films by using, forexample, co-evaporation [5], molecular-beam epitaxy (MBE)[6], and hybrid physical-chemical vapor deposition (HPCVD)[7], [8], [9] methods. The fabrication of MgB2-based SSPDsand HEB mixers has also been demonstrated and encouragingresults have been obtained [10], [11], [12].

For practical device applications, the stability of MgB2

ultrathin films under service environment is important to knowas it would determine the time over which the device couldmaintain its reliable performance. Previously there have beenstudies on the degradation of MgB2 thin films and bulksamples with exposure to water [13], [14], [15] or to ambientenvironment [16]. In the studies of Zhai et al. [13] and Cui etal. [15] on MgB2 films in thicknesses above 100 nm, it wasfound that both the zero-resistance transition temperature Tc0and the thickness of the films decreased with increasing thetime of water exposure. The sensitivity of MgB2 to water wasalso investigated by Cheng et al. [14] for bulk polycrystalsand an exponential decay of the superconducting propertiessuch as the diamagnetic magnetization with exposure time wasobserved. Under ambient condition, namely exposing to air,Serquis et al. [16] compared the degradation of MgB2 bulksamples prepared by different processes and found that thepoorly-sintered samples with smaller grain size degraded withair-exposure time while the well-sintered samples with largergrains remained largely unchanged. It was further suggestedthat the degradation of MgB2 in air may be related to a surfacedecomposition with Mg oxy-hydroxide forming in the surfacelayer of the sample.

In this paper we present a comparative study on the stabilityof MgB2 ultrathin films under three different environmentswith varied humidity and atmospheric pressure, that is, inair, in desiccator, and in low vacuum. The MgB2 films withthicknesses about 15 nm were grown by the HPCVD method.It was found that, while the films exposed to air showed appar-ent degradation by increasing the storage time beyond aboutone month, the films placed in desiccator or in low vacuumexhibited much less degradation and particularly the film in




low vacuum remained reasonably stable in the investigatedperiod of time (∼ 4 months). This contrast suggests that themoisture in the air should be the leading cause of degradationof MgB2 ultrathin films under ambient environment. Theresults also show that MgB2 ultrathin films can maintain thelong-time stability in a dry air or preferably a vacuum or anN2 environment, which demonstrates the prospect of the filmsin practical superconducting applications.

II. EXPERIMENTAL DETAILS

The HPCVD method utilized to grow the MgB2 ultrathinfilms has been described in detail elsewhere [17], [18]. Duringthe present deposition process [9], [19], there was a flow ofpurified H2 gas through the reactor with a total pressure of ∼ 5kPa to prevent oxygen contamination of the film. The substratewas placed on the top surface of a susceptor which was heatedresistively to the deposition temperature of 650 − 700 C.Around the substrate and also on the top of the susceptor wereseveral Mg slugs, whose evaporation at elevated temperaturesprovided the Mg source and the high Mg vapor pressure nearthe substrate. When a diborane (B2H6) gas mixture (5% in H2)was introduced into the reactor the thermal decomposition ofB2H6 near the substrate provided the purified boron sourceand hence the MgB2 film growth was initiated. The thicknessof the film was controlled by varying both the B2H6 supplytime and its flow rate. In the present study, ultrathin MgB2

films were grown on 2 × 10 mm2 substrates of (111) MgOwith deposition time of 90 s and B2H6 flow rate of 1.9 sccm(standard cubic centimeter per minute). The thicknesses ofthe films were ∼ 15 nm as indicated by the atomic forcemicroscopy (AFM) measurements on films grown with thesame growth parameters.

For the present study six MgB2 ultrathin films were pre-pared under identical deposition conditions during three sep-arate runs, with each two films grown at the same time. Tocompare the stability of the films in various environments, weplaced them under three different environmental conditions: inair, in desiccator, and in low vacuum. Under each environmentthere were two films; one was for the resistance measurementsand the other for the supplemental scanning electron mi-croscopy (SEM) and X-ray diffraction (XRD) measurements.All films were placed at room temperature around 25 C. Forthe films stored in air, namely in ambient environment, therelative humidity of the air was in the range of 24% − 30%,which was recorded by a hygrometer (TH101B, Anymetre)placed near to the films. In comparison, for the films storedin a desiccator with allochroic silicagel, the relative humiditywas ∼ 6%, that is, 4 − 5 times lower than that of ambientenvironment. Furthermore, for the films kept in a vacuumchamber, while the relative humidity was roughly the same asthat in desiccator, the air pressure was reduced to be 5×104 Pa,i.e. about half the standard atmospheric pressure, by vacuum-pumping the chamber for a few minutes (as the vacuum wasnot high, it could be maintained well for a long time withoutcontinuously pumping the chamber). Hereafter we label thefilms stored in air, desiccator, and low vacuum as sample #1,sample #2, and sample #3, respectively.

To investigate the degradation of the films, the temperature(5− 300 K) dependence of the resistance under various mag-netic fields (0 − 8 T) was measured at different storage times(between 1 and 115 days), from which the time evolutions ofthe normal-state resistance, the Tc, and the upper critical fieldHc2 of the films were extracted. The resistance was measuredusing the standard four-probe method in linear configurationin a physical property measurement system (PPMS, QuantumDesign), with the magnetic field perpendicular to the filmsurface, that is, parallel to the c-axis of the film. In addition,SEM images and XRD patterns of the films were also taken atdifferent storage times. The secondary electron SEM imageswere measured with a field emission scanning electron mi-croscope equipped with a through-the-length detector (NovaNanoSEM 430, FEI), while the XRD patterns were recordedby using an X-ray diffractometer (D/max-RA, Rigaku) withCu Kα radiation and a scan length of 20-90 (2θ) at 4/min.

III. RESULTS AND DISCUSSION

Fig. 1 shows the sheet resistance Rs (resistance per square)versus temperature for the films under three environmentalconditions after different storage times. As mentioned earlier,in previous studies of the degradation of MgB2 films in water[13], [15], it was indicated that the thickness of the filmdecreased with increasing the time of water exposure. Thisreduction of the effective MgB2 layer thickness has also beensuggested to happen for MgB2 bulk samples degrading inair as a result of a surface decomposition [16]. These pointsreminded us that, when investigating the degradation of MgB2

ultrathin films, the possible variation of the effective filmthickness d as a function of the storage time had to be takeninto account. Hence, in the present study we have not chosento plot the resistivity ρ, but instead plotted the Rs = ρ/dto recognize the possible changes in both ρ and d of thefilms with varying the time. The individual variation of ρor d with time could be indicated in the time evolutionsof other properties of the films such as the Hc2 and SEMimages discussed below. Here we should also point out that,although our first set of resistance data of the three sampleswas recorded at one day after the film growth, we believe itcould also be treated as the set of data for as-grown films.This was indicated by the finding that the room-temperatureresistance of the as-grown films remained unchanged after thefilms storing for one day in different environments.

In the main panels of Fig. 1, it is seen that the Rs(T ) curvesabove Tc shifted upward as the time increased for all the threesamples, suggesting the presence of an aging effect in all cases.It is evident, however, that the growth rate of Rs(T ) with timeis not uniform for a given sample and differs among the threesamples. For sample #1, as shown in Fig. 1(a), initially theincrease in Rs(T ) was rather small when the storage time waswithin 35 days, but it became large and significant as the timewas further increased from 35 to 115 days. For sample #2, ascan be seen in Fig. 1(b), while the growth of Rs(T ) showeda similar time evolution to sample #1, the magnitude of thegrowth was considerably smaller. Compared with sample #1and sample #2, Fig. 1(c) shows that sample #3 exhibited the




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Fig. 1. Sheet resistance vs. temperature curves of MgB2 ultrathin filmsobtained after different storage times. Shown in (a), (b), and (c) are the MgB2

films in thicknesses about 15 nm stored in air (sample #1), in desiccator(sample #2), and in vacuum (sample #3), respectively. Insets show the curvesin the vicinity of the superconducting transition.

least increase in Rs(T ) with increasing time. It is impressiveto see that for sample #3 the Rs(T ) barely changed as the timewent on from 77 to 115 days. Therefore, Fig. 1 demonstratesthat the extent of the degradation was different among the threesamples placed under three different environmental conditions,at least in terms of the normal-state resistance.

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Fig. 2. Time evolution of the change in sheet resistance from 300 to40 K, ∆Rs = Rs(300 K) − Rs(40 K) (a), and the residual resistanceratio, RRR = Rs(300 K)/Rs(40 K) (b), of MgB2 ultrathin films storedunder three different environmental conditions, namely in air (sample #1), indesiccator (sample #2), and in vacuum (sample #3).

To better compare the time evolution of Rs(T ) of thethree samples, we plot in Fig. 2 the time dependence ofthe sheet resistance difference ∆Rs and the residual resis-tance ratio RRR, which are usually used to characterize thenormal-state properties of superconducting thin films. Here∆Rs = Rs(300 K)−Rs(40 K) defined as the difference in thesheet resistance between 300 and 40 K, that is, between theroom temperature and the temperature just above Tc, whileRRR = Rs(300 K)/Rs(40 K). From Fig. 2(a) it is seenthat at the beginning of the storage the ∆Rs of sample #2and sample #3 were nearly the same, which, together withother nearly identical film properties such as the Tc shownbelow, suggest good reproducibility of the film growth betweenthe two runs. For sample #1, the ∆Rs was slightly higher,presumably due to some small differences in the film-growthconditions from sample #2 and sample #3. As the storage timeincreased to 35 days, we can see that the ∆Rs showed similarbehavior for all the films and increased only slightly. Withfurther increasing the time, however, the ∆Rs started to showdifferent behavior among the three samples. As can be seen,for sample #1 the ∆Rs began to increase very strongly withtime, becoming nearly four times large as the time went onfrom 35 to 115 days. For sample #2, the increase in ∆Rs alsobecame a little bit stronger, which is similar to sample #1, butthe overall increase was kept small. In comparison, the ∆Rs

for sample #3 remained little changed throughout the wholetime of storage. The above contrast among the three samples




was also found in the time variation of RRR, as shown in Fig.2(b). It is seen that over the whole storage time the behaviorof the reduction in RRR was not the same for the films storedin different environments: while the decrease of RRR seemedto be similar for the three films as time increased to about35 days, with time further increasing the RRR of sample #1and sample #2 decreased faster than that of sample #3. Thisindicates that the degradation of the three films start to becomedifferent when the time went beyond 35 days, as manifestedin the time evolution of ∆Rs shown in Fig. 2(a).

The above comparison suggests that for sample #3 placedunder low vacuum the degradation remained slow in the wholeinvestigated period of time, while for sample #2 kept indesiccator and particularly for sample #1 with exposure toair the degradation showed a change at the time of aboutone month and started to become faster as the time furtherincreased since then. For MgB2, it has been proposed thatthe change in the measured resistivity from 300 to 40 K,∆ρm = ρ(300 K) − ρ(40 K), could be used to evaluatethe connectivity of the sample with ∆ρm = F∆ρg, where1/F is the fractional area of the sample that carries currentand ∆ρg ∼ 7 − 9 µΩcm is the corresponding change inresistivity for a fully-connected MgB2 sample [2], [20], [21].From ∆Rs = ∆ρm/d and d ∼ 15 nm, we can see from Fig.2(a) that initially the ∆ρm of the three films were close to∆ρg, indicating nearly full connectivity of the films. As timeincreased, the increase of ∆Rs could be due to an increase of∆ρm, which, according to the above, implied a reduction ofconnectivity of the film, and/or a decrease of d. This suggeststhat, when the storage time was beyond about one month,the much faster degradation of sample #1 than sample #2and sample #3 in terms of the quite larger increase of ∆Rs

might correspond to a much stronger decline of either theconnectivity or the effective film thickness or both of sample#1. For instance, if we assume the effective film thickness tobe constant, the connectivity of sample #1 would decreaseby about 73% when time increased from 35 to 115 days,as inferred from the increase of ∆Rs, while for sample #2and sample #3 the corresponding decrease in connectivity wasconsiderably lower, at about 31% and 5%, respectively.

The RRR has been widely referred to indicate the strengthof impurity scattering in the sample: the lower the RRR, thehigher the impurity or disorder level of the sample. Accordingto this notion, the decrease of RRR with increasing timefor the three samples, as shown in Fig. 2(b), suggests thegeneration of new impurities or disorder in the films as theydegraded with time. As pointed out earlier, when the storagetime was beyond about 35 days, the RRR of sample #1and sample #2 started to decrease faster than that of sample#3. With time going from 35 to 115 days, the RRR wasfound to decrease by about 29% and 18% for sample #1and sample #2 respectively, while it decreased by only about5% for sample #3. This contrast further suggests the role ofair exposure in introducing impurities in the films. Moreover,it is noted that the RRR and ∆Rs could be related byRRR = 1 + ∆Rs/Rs(40 K), which shows that, as the ∆Rs

increased with increasing time, it was the stronger increaseof Rs(40 K), i.e. the residual sheet resistance, that led to the

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Fig. 3. Time dependence of the superconducting critical temperature, Tc, ofMgB2 ultrathin films stored in air (sample #1), in desiccator (sample #2), andin vacuum (sample #3).

decrease of RRR with time. It also shows that, as the increaseof ∆Rs for sample #1 was much larger than that for the othertwo samples when time increased from 35 to 115 days, theincrease of Rs(40 K) was much larger as well for sample#1 over the same period of time. This is in accordance withwhat we have seen in Fig. 1 and further suggests that themoisture in the air should play an important role in causingthe degradation of the film with the increase of impurities inthe sample.

After examining the normal-state Rs(T ), we now proceed toexplore the time evolution of the superconducting properties ofthe three samples. In insets of Fig. 1, we have shown the Rs(T )curves of the films in the vicinity of the superconductingtransition. From the inset of Fig. 1(a) it is seen that for sample#1 the occurrence of the superconducting transition shifted tolower temperatures as the storage time increased, showing thedecline of Tc with increasing time. In comparison, insets ofFigs. 1(b) and 1(c) show that for sample #2 and sample #3such downward shift of the superconducting transition becameprogressively less pronounced. To quantify the analysis, wehave defined the Tc of the sample as the midpoint of thesuperconducting transition in each Rs(T ) curve and plottedits time evolution for the three films in Fig. 3, where theuncertainty of the Tc, i.e. ∆Tc, has been determined as thetransition width from 90% to 10% of the resistance drop. Itcan be seen that initially the Tc of sample #1 was about 36 K,while the Tc of both sample #2 and sample #3 were about 36.5K, i.e. about 0.5 K higher than that of sample #1. One may notethat these Tc values, while typical for HPCVD-prepared MgB2

ultrathin films of similar thicknesses [7], [8], [9], were a littlesmaller than the value of ∼ 39 K in MgB2 bulk samples orthick films [1], [2]. As discussed in detail in our previous study[19], this weak suppression of Tc in ultrathin MgB2 films mayarise from the increase of impurity scattering or disorder levelin the film as the film thickness reduces towards the ultrathinregime. This explanation may account for the slightly lower Tcof sample #1 than the other two samples as the disorder level




of this film was slightly higher as indicated by the Rs(40 K)and RRR shown respectively in Figs. 1 and 2. Recently itwas shown that in-plane compressive strain in MgB2 filmsinduced by the substrates could also lead to Tc of the filmslower than the bulk value [22]. For the present ultrathin films,we found that the c-axis lattice constant deduced from theXRD experiment (shown below in Fig. 6) was ∼ 3.521 A,which was slightly lower than the value of 3.524 A in bulkMgB2 and probably indicated an in-plane tensile, rather thancompressive, strain in the films [23].

With increasing time, Fig. 3 shows that the Tc of sample#1 decreased gradually, corresponding to the shift of thesuperconducting transition shown in inset of Fig. 1(a). The∆Tc also became a little bit larger, suggesting larger widthof the superconducting transition. Compared to sample #1,the decrease of Tc with increasing time is shown to beconsiderably smaller for both sample #2 and sample #3. Wecan see that for these two samples the variation of Tc wasnearly the same as time increased to 35 days and with timefurther increasing the Tc of sample #2 became slightly lowerthan that of sample #3. It is remarkable to find that over thewhole time of storage (115 days) the decrease in Tc wasjust about 0.18 and 0.09 K for sample #2 and sample #3respectively, in contrast to the value of 0.5 K for sample #1.It could also be seen that for time increasing to 23 days thedecrease of Tc of sample #1 was actually slow and similar tothat of sample #2 and sample #3, but as the time increasedfurther the Tc of sample #1 started to decrease faster thanthat of the other two samples. This suggests the presence of achange in the degradation of sample #1 when the storage timearriving at about one month, as indicated above in the timeevolution of ∆Rs and RRR of the sample shown in Fig. 2.

Although the decrease of Tc in sample #1 was the strongestamong the three samples, which suggests again the linkbetween the degradation of MgB2 ultrathin films and themoisture in the air, it is worth noting that the total Tc declineof this sample over the whole storage time of 115 days was notsignificant, only around 0.5 K as mentioned earlier. This rathersmall Tc degradation, corresponding to just 1.4% decrease ofTc, may not be expected by considering the long time of airexposure and, in particular, the large increase of Rs(40 K) and∆Rs of the sample as shown in Figs. 1(a) and 2(a) respectively,which, as discussed above, implied large increase of impurityscattering and decrease of connectivity of the sample. It seemsthat these effects had no significant impact on the Tc of thesample. In regard to impurity scattering, one may note thatthere are two scattering channels, intraband and interbandscattering, in the two-band superconductor MgB2 and onlythe interband scattering could lead to Tc suppression of thesample. It could be possible that for sample #1 the largeincrease of Rs(40 K) with time arose mainly from the increaseof intraband scattering, which then had little effect on the Tcof the sample [19].

We have also determined the upper critical field Hc2 ofthe samples as a function of the storage time to gain moreinsight into the degradation of MgB2 ultrathin films. TheHc2 of the sample was determined from measurements of thesuperconducting transition under different magnetic fields. To

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Fig. 4. (a) Temperature dependence of the upper critical field (symbols),Hc2, of MgB2 ultrathin films stored in air (sample #1), in desiccator (sample#2), and in vacuum (sample #3) for one day after growth of the films. Dottedlines are linear fits to the Hc2, which are used to extract the value of Hc2 atzero-temperature limit, Hc2(0). Inset shows temperature dependence of theRs in different magnetic fields for sample #1 stored in air for one day. (b)Hc2(0) of the three MgB2 ultrathin films as a function of time.

illustrate this, in inset of Fig. 4(a) we show as an examplethe Rs(T ) curves in fields of 0 − 8 T for sample #1 storedin air for one day. It can be seen that the superconductingtransition occurred at lower temperatures with increasing fieldsand eventually disappeared in the measured temperature rangewhen the field was high enough. By defining the point atwhich the Rs(T ) became 99% of Rs(40 K) as the onsetof the superconducting transition in each field, the Hc2 ofsample #1 at different temperatures were extracted, as shownin the main panel of Fig. 4(a) where similar data for sample#2 and sample #3 with one-day storage are also displayed. Itis seen that the Hc2(T ) of the three samples exhibited lineartemperature dependencies, as usually observed in clean MgB2

single crystals [24] and thin films [25]. Accordingly, we havedetermined the zero-temperature Hc2 of the samples, Hc2(0),through linear fits to the Hc2(T ) data, as shown by dottedlines in the main panel of Fig. 4(a). It was found that thusdetermined Hc2(0), when it being below 8 T, was consistentwith that might be indicated from the Rs(T ) measurements atrelatively high fields. For instance, as seen in the main panel




of Fig. 4(a), the Hc2(0) was determined to be about 6.1 Tfor sample #1 after one-day storage, which is in accordancewith what we have seen in the inset of Fig. 4(a) that thesuperconducting transition of the sample was no longer seenin the Rs(T ) curves when the field was above 6 T.

The time dependence of Hc2(0) has been summarized forthe three samples in Fig. 4(b). We can see that initially thethree samples showed similar Hc2(0), which was in the rangeof 5 − 7 T suggesting the films were relatively clean [24],[25]. As time increased, the Hc2(0) of sample #1 increasedrather steeply, becoming about three times larger for timeincreasing to 115 days. In comparison, for sample #2 andsample #3 the increase in Hc2(0) with increasing time wasrather modest, within 20% over the whole storage time. Thiscontrast in the time variation of Hc2(0) reaffirmed the muchstronger degradation of sample #1 with exposure to air thanthe other two samples as perceived above from the contrastof time evolution of other properties such as the ∆Rs and Tcshown in Figs. 2 and 3 respectively. Moreover, it is known thatHc2(0) depends on the purity of the sample and specificallyHc2(0) = 2.77

π ekBNFρnTc as calculated for conventionalsingle-gap superconductors in the dirty limit, where e is theelectron charge, kB is Boltzmann constant, NF is the density ofstates at the Fermi level, and ρn is the normal-state resistivityjust above Tc [26], [27]. For two-gap superconductor MgB2,although the above simple relation between Hc2(0) and ρn isusually not applicable due to the presence of multiple impurityscattering channels involving both intraband and interbandscattering [28], the qualitative, positive correlation betweenHc2(0) and ρn, namely ρ(40 K) in MgB2, still holds. This tellsus that the increase of Hc2(0) shown in Fig. 4(b), particularlyfor sample #1, actually implied the increase of ρ(40 K) withincreasing time. In other words, the degradation of the samplesas reflected by the increase of Hc2(0) should be connectedwith proliferations of impurities or disorder in the films, in linewith the suggestion made above from the direct observationof the increase of Rs(40 K) as shown in Fig. 1.

To further probe the degradation of the films, we havealso taken SEM images and XRD patterns of the samples atcertain time intervals to get information complementary to thatobtained from the resistivity measurements. Fig. 5 comparesSEM images of the three samples after storing for one day(left panels) and 115 days (right panels). As seen from Figs.5(a) to 5(c), the three films had similar surface morphologiesat the beginning of the storage, showing surface terraces withsharp edges formed by the close connection and stacking ofplate-like grains in different heights. There were also discretehexagonal-shaped grains shown on the very top of the filmsurfaces. These suggest the epitaxial growth of the films [9].It may also be seen that, the c-axis of the plate-like grainsseems to be slightly tilted from the substrate normal, similarto the observation in the growth of MgB2 films on some othersubstrates [25], [29]. We note that in our previously grownultrathin MgB2 films on MgO substrates this phenomenonof tilted growth was not present [9]. Hence the reason forits presence for the current films, either intrinsic owing tothe lattice mismatch between the film and the substrate [25],[29] or extrinsic owing to a very small misalignment of the

Fig. 5. Scanning electron microscopy (SEM) images of MgB2 ultrathin filmsstored under three different environmental conditions: in air (sample #1) [(a)and (d)], in desiccator (sample #2) [(b) and (e)], and in vacuum (sample #3)[(c) and (f)]. (a), (b), and (c) show the films stored for one day, while (d),(e), and (f) show the same films stored for 115 days.

substrate itself, needs to be further checked. After storage for115 days under different environments, Figs. 5(d) to 5(f) showthat the morphologies of the three films changed differently.For sample #1, Fig. 5(d) shows that, in contrast with Fig.5(a), the apparent surface terraces were no longer seen andmoreover there appeared to be voids generated and dispersedin the surface. This contrast suggests the decrease of boththe film thickness and the film connectivity, correspondingwell with the indication from the large increase of ∆Rs insample #1 as shown in Fig. 2 and discussed above. Forsample #2, Fig. 5(e) shows that while the surface terracesalso became largely blurred compared with Fig. 5(b), theconnectivity of the film remained intact, pointing to a lessdegradation of sample #2 than sample #1. For sample #3,Fig. 5(f) shows that the surface terraces could still be readilyobserved. The main difference from Fig. 5(c) may lie in thatthe edges of the terraces became rounded and less sharp,suggesting the corrosion of grain boundaries as the main formof sample degradation. This also indicates that among the threesamples the degradation in sample #3 should be the smallest,in agreement with the conclusion drawn from the resistivitymeasurements. Here we may note that the above different timeevolutions of SEM images of the three films actually providea morphological viewpoint to evaluate the effects of exposingto different environments on the devices such as SSPDs and




MgB 2 (0

001)

Subs

trate

Subs

trate

MgB 2 (0

002)

1 1 5 d a y s

Inten

sity (a

rbitra

ry un

its)

2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0

i n a i r ( S a m p l e # 1 )

i n d e s i c c a t o r ( S a m p l e # 2 )

i n v a c u u m ( S a m p l e # 3 )

2 θ ( d e g r e e s )

Fig. 6. X-ray diffraction (XRD) patterns (Cu Kα radiation) of MgB2 ultrathinfilms stored in air (sample #1), in desiccator (sample #2), and in vacuum(sample #3) for 115 days after growth of the films.

HEBs patterned from the ultrathin MgB2 films. For SSPDs andHEBs, key elements are micro-bridges or wires in sub-micronscale. Fig. 5(d) indicates that the homogeneity or intactnessof the bridges or wires, and consequently the performance ofthe devices, could be appreciably affected when placing thedevices in ambient environment for a long time. By storingthe devices in desiccator or low vacuum, the situation maybecome much better as the intactness of the bridges or wirescould be preserved as suggested by Figs. 5(e) and 5(f).

The XRD patterns of the three samples after storing for115 days are shown in Fig. 6. It is seen that, besides thediffraction peaks from the (111) MgO substrate, an MgB2

(0002) peak appeared for each of the three samples. Thissuggests the predominant c-axis orientation of the films, inagreement with the SEM images discussed above. For sample#3, there is also a small MgB2 (0001) peak appearing in thespectrum, as usually observed for fresh MgB2 ultrathin filmsat similar thicknesses on MgO substrates [9]. In contrast, wesee that this MgB2 (0001) peak seems to be absent in thespectra of sample #1 and sample #2. Moreover, it is noted thatthe intensity of the MgB2 (0002) peak, which is expected tobe similar for the three samples before starting the storage,is apparently weaker in sample #1 and sample #2 than insample #3. These observations suggest that, after storage for115 days, the peak intensity which is indicative of the qualityof the MgB2 phase decreased in sample #1 and sample #2.We note that, in a previous study of the degradation of MgB2

bulk pellets under air exposure, similar phenomenon was alsoreported by Serquis et al. [16]. By using X-ray photoelectronspectroscopy they further showed that the degradation of thesample was caused by a surface decomposition with Mghydroxide Mg(OH)2, MgO, and also possible Mg oxycarbideforming in the surface layer of the sample [16]. It is likely thatsimilar degradation processes occurred for the present MgB2

ultrathin films of sample #1 and sample #2, thereby resultingin the decrease of the intensity of MgB2 phase in them. Notethat this also implies the generation of impurities in the filmsduring the storage, consistent as well with the above resistivitymeasurements. On the other hand, it is seen from Fig. 6 that

no apparent diffraction peaks associated with impurities seemto be present in the XRD spectra of sample #1 and sample#2. This may be due to the limited resolution of the XRDmeasurement and the very small amount of the impurities asthe films themselves are very thin.

In the above, by means of resistivity, SEM, and XRD mea-surements, we have comparatively investigated the stability ofthree MgB2 ultrathin films placed under three different envi-ronmental conditions: air, desiccator, and low vacuum. Basedon these experiments, some aspects concerning the degradationof the three samples may be summarized as follows. First, aswe have seen from Figs. 1 to 4, when the storage time waswithin about one month, all the three films remained relativelystable with similarly slow degradation; when time went beyondthis value, however, the degradation of the three films startedto become different: although the degradation of sample #3remained slow, the degradation of sample #1 and sample #2became faster and particularly the degradation of sample #1became much larger than that of sample #2 and sample #3.This suggests that it was after a certain storage time that thefilms placed in air or desiccator began to degrade significantly.Second, over the whole storage time, the different extentsof degradation of the three samples reflect different mannersof film degradation in them. For sample #1, the degradationinvolved the generation of impurities or disorder in the film,the decline of film connectivity, and the reduction of effectivefilm thickness, as indicated respectively by the large increaseof Rs(40 K) (Fig. 1) and Hc2(0) (Fig. 4), the large increase of∆Rs (Fig. 2), and the evolution of SEM images (Fig. 5) withincreasing time. In comparison, the degradation of sample #2may be mainly confined to some decomposition at the filmsurface as suggested by SEM and XRD experiments (Figs.5 and 6), thus giving rise to much smaller time-variation inthe resistive transport as shown in Figs. 1 to 4. For sample#3 showing the least degradation among the three samples,SEM images (Fig. 5) suggest that the degradation was furtherreduced to be corrosion of grain boundaries at the surface ofthe film. Third, comparison of the degradation of the threesamples suggests that the moisture in the air may play theleading role in causing the degradation of MgB2 ultrathinfilms under ambient environment. This follows from the muchsmaller degradation of sample #2 than sample #1 and the factthat the main difference in the surrounding environments ofthe two samples rests with the humidity in desiccator forsample #2 considerably lower than that in air for sample#1. This conclusion is consistent with previous studies whichindicated the sensitivity of MgB2 to water [13], [14], [15] andin particular the study showing that the degradation of MgB2

bulk sample in air was manifested primarily by the formationof Mg hydroxide at sample surface as a result of the reactionbetween sample and the water vapor in the air [16]. Accordingto this study, presence of Mg oxide and carbonate in surfacelayer of the sample also contributed to the degradation ofMgB2 owing to the reaction of sample with oxygen and CO2

in the air. This may account for the further smaller degradationof sample #3 than sample #2 observed in the present study,as sample #3 was placed in a vacuum chamber with reducedatmospheric pressure and consequently had less chance than




sample #2 to react with the gases in the air.

IV. CONCLUSION

In summary, we have performed a comparative study on thedegradation of MgB2 ultrathin films of thicknesses of about 15nm, fabricated via the HPCVD method, by placing them underthree different environments: air, desiccator, and low vacuum.By using resistivity together with SEM and XRD measure-ments, we have determined the normal-state resistance, thesuperconducting properties such as the Tc and Hc2, and thesurface morphologies of the films as a function of time overa span of about 4 months. Collectively, it has been foundthat, for sample #1 in ambient air, the apparent degradationstarted to show for time greater than about one month, whichwas manifested in the increased impurities or disorder in thefilm and the decreased connectivity and effective thickness ofthe film. In comparison, for sample #2 in a desiccator withreduced humidity, although the degradation of the film alsobecame faster as time increased beyond about one month,the extent of the degradation was much smaller than thatof sample #1 in air, suggesting that, similar to the case ofMgB2 bulk samples, the degradation of MgB2 ultrathin filmsin ambient environment mainly result from the interaction ofthe sample with the moisture in the air. With further reducingthe atmospheric pressure, sample #3 in low vacuum was foundto show the least degradation among the three samples andremained sufficiently stable over the whole investigated periodof time. These results demonstrate that in dry air or preferablyvacuum environments ultrathin MgB2 films can maintain long-term stability. In practical applications, one may also putMgB2 ultrathin films in an N2 or Ar environment in place ofa vacuum environment to prevent degradation. Alternatively,one may coat the MgB2 thin film with a protective layerwhich would also prevent contact with ambient air and preventdegradation of the film. In this case, the protective layer,preferably being transparent insulating materials, should beas thin as possible to avoid impairing the performance of thedevices like SSPDs and HEBs made from the film.

ACKNOWLEDGMENT

The authors would like to thank Xiao-Jing Wang and Hua-Bo Zhao for invaluable experimental help.

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Degradation of MgB 2 Ultrathin Films Under Different Environmental Conditions