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ساختار Biepitaxid، پيوند josephson و SQUIDs : ساختار و ويژگي‌هاي اتصال يا پيوند josephson و SQUIDs، yBCo/CeO2/Mgo Biepitaxid را بررسي و گزارش كرديم، در اينجا CeO2 به عنوان يك لايه استفاده شده تا يك محدوده يا مرز Biepitaxid براي بلور يا ذره ايجاد كند. سطوح تابكاري نشده لايه CeO2 و سطح تابكاري شده آن توسط ميكروسكوپ اتر يا (AFM) بررسي شدند. دماي مقاومت لايه نازك yBCo/CeO2/Mgo نشان مي‌دهد كه فرآيند تابكاري براي لايه CeO2 به منظور ايجاد لايه فيلم YBCo با كيفيت خوب ضروري است. منحني ولتاژ جريان اتصال يا پيوند josephson عملكرد مربوط به مقاومت اتصال (RSJ) را نشان مي‌دهد. بعلاوه هر دو مرحله عددي يا انتگرال و يا نيمه انتگرال Shapiro در ميدان مغناطيسي بكاربرده شده صفر مشاهده شد نوسان ولتاژ مغناطيسي مدولي شده نيز براي SQUIDs ديده مي‌شود.
1- مقدمه: بدليل توسعه و پيشرفت مدارهاي مجتمع ابررسانا، High-T، اتصالات ابررساناي josephson به شكل گسترده‌اي مورد بررسي و آزمايش واقع شدند. به منظور بدست آوردن اتصالات josephson قابل كنترل و قابل دستيابي به انواع مختلف اتصالات مانند محدوده داراي لبه پله‌اي، SNS يا اساس و پايه bicrystaf استفاده شد. بهرحال اين نوع مرزها معمولاً طي زمان ساخت با فرآيندهاي بسيار زيادي درگير هستند. براي ساده‌تر كردن فرآيند ساخت، اتصالات josephson محدوده دانه Biepitaxid مورد بررسي واقع شد. در اين كار، CeCo2 انتخاب شد تا يك لايه براي محدوده ذره Biepitaxid باشد و روش ساده و شيميايي حكاكي بجاي فرزكاري آهن استفاده شد تا باعث جدا شدن نيمي از لايه CeCo2 از پايه Mgo شود.
SQUIDs و اتصالات josephson Biepitaxid را ساختيم. بعضي از ويژگي‌هاي جريان ولتاژ براي اتصالات josephson و SQUIDs مورد بررسي واقع شد. همچنين نوسان ولتاژ تقسيم شده در ميدان مغناطيسي براي SQUIDs نيز بررسي شد.
2- شرح تجربي و آزمايشي: يك سيستم آبكاري فلز مغناطيسي rf خارج از محور براي جدا كردن تمام لايه‌ها در اين مبحث استفاده شده است. CeCo2 در دماي OC750 برروي سطح Mgo كه با يك لايه (yBCo)800 A-thick پوشيده شده، آبكاري شد. سپس لايه yBCo / CeCo2 با استفاده از اسيد هيدروكلريك جدا شد. بعد از آن، اين پايه و اساس در دماي OC1100 به مدت 10 ساعت تابكاري شد. براي بررسي تغييرات سطح CeCo2، ساختار سطحي لايه‌هاي CeCo2 تابكاري شده و تابكاري نشده با ميكروسكوپ‌هاي (AFM) بررسي شدند و سپس يك لايه (yBCo)2000 A-thick برروي سطح تابكاري شده قرار داده شد، رسوب‌گذاري شده بعلاوه لايه نازك yBCo با يك محدوده يا مرز Biepitaxid توسط فتوليتوگرافي در يك اتصال josephson با 5pm پهنا يا SQUIDs با يك ناحيه سوراخ 40*20 و پهناي اتصال بصورت طرح و نقش قرار داده شد. براي بررسي واكنش‌هاي ميكرو ويوي اتصال، يك ميكرو ويو با استفاده از آنتن ديود به اين اتصال تابانده شد. براي بررسي نوسان بخش بخش ميدان مغناطيسي، SQUID برروي يك سولئوئيد كه داراي ميدان مغناطيسي موازي با سطح SQUID بود نصف شد. يك روش معمولي چهارمرحله‌اي براي اندازه‌گيري و بخش الكتريكي استفاده شد. معيار براي جريان اصلي در اين مبحث بود.
شكل 1- مقاومت دماي لايه نازك yBCo برروي a، سطح Mgo، b، سطح تابكاري نشده Mgo / CeCo2.
3- بحث و نتيجه‌گيري: بخ منظور تأييد ويژگي خوب و مناسب لايه نازك yBCo ، مقاومت دمائي لايه نازك yBCo برروي پايه Mgo و لايه CeCo2 تابكاري نشده همانند شكل 1 اندازه‌گيري شد. لايه yBCo برروي پايه Mgo عملكرد ابررسانايي با نشان مي‌دهد. بهرحال لايه yBCo برروي لايه CeCo2 تابكاري نشده مقاومت نيمه رسانايي با افت دماي حدود k82 را نشان مي‌دهد. اين مقاومت نيمه رسانايي ممكن است بدليل عنوان شده در زير باشد. ابتدا، تركيب yBCo تغيير مي‌كند، تا برروي لايه CeCo2 تابكاري نشده آبكاري شود. ثانياً بلور يا ذره yBCo همسطح نمي‌باشند. براي بررسي اوليه نقطه، انحراف يا خمش پرتو x براي مورد نمونه تابكاري نشده و مشخص مي‌شود و در شكل 2 نشان داده شده است. مشاهده مي‌شود كه تمام سطوح {o,o,n} براي yBCo و CeCo2 واضح و مشخص هستند. اين موضوع نشان مي‌دهد كه ساختار و تركيب لايه‌هاي CeCo2 و yBCo دقيق و درست است و اولين مورد ويژه و اختصاصي است. سپس سطح لايه نازك yBCo برروي لايه تابكاري نشده CeCo2 با ميكروسكوپ اتمي (AFM) بررسي مي‌شود كه در شكل 3 نشان داده شده است.
Fabrication of Biepitaxial YBazCu307_,
Josephson Junctions and SQUIDS
S. Y. Yang?, H. E. Horngl, W. L. Lee2, H. W. Yu2, and H. C. Yang2
1 Department of Physics, National Taiwan Normal University, Taipei, Taiwan 117, R.O.C.
2Department of Physics, National Taiwan University, Taapei, Taiwan 106, R.O.C.
(Received December 20, 1997)
We reported the fabrication and characteristics of biepitaxial YBCO/CeOz/MgO
Josephson junctions and SQUIDS, here Ce02 was used as a seed layer to create the biepitaxial
grain boundary. The surfaces of the unannealed CeO2 layer and the annealed
one were probed by the atomic force microscope (AFM). The temperature dependent
resistance of YBCO/Ce02/MgO thin film reveals that the annealing process for CeO2
layer is crucial for the quality of the YBCO thin film. The voltage-current curves
of Josephson junctions exhibit the resistively shunted junction (RSJ) behavior. Furthermore,
both the integral and half-integral Shapiro steps were observed under zero
applied magnetic field. The magnetic modulated voltage oscillation was also found for
the SQUIDS.
PACS. 74.50.+r - Proximity effects, weak links, tunneling phenomena, and Josephson
effects.
_ PACS. 74.76.-w - Superconducting films.
I. Introduction
Owing to the development of the superconducting integrated circuits, high-T, superconducting
Josephson junctions have been examined widely. In order to obtain the
controllable and reproducible Josephson junctions, various types of junctions are used,
such as step-edge boundary [l], SNS [2] or bicrystal substrate [3]. However, these kinds
of boundaries usually involve too many processes during fabrication [4,5]. To simplify the
fabrication processes, biepitaxial grain boundary Josephson junctions were studied. In this
work, CeO2 was chosen to be a seed layer for the biepitaxial YBazCusO7_, grain boundary
and an easy chemical etching method instead of conventional ion milling was used to lift
off half of the CeO2 layer on MgO substrate. We fabricated the biepitaxial YBa2Cu307_y
Josephson junctions and SQUIDS. Some voltage-current characteristics for the Josephson
junctions and SQUIDS were investigated. And also, the magnetic field modulated voltage
oscillation for the SQUIDS was checked.
II. Experimental details
An off-axis rf magnetron sputtering system was used to deposit all films in this work.
Ce02 was sputtered at 750 ?C onto the MgO(001) substrate which was covered partly by
409 @ 1998 THE PHYSICAL SOCIETY
OF THE REPUBLIC OF CHINA
410 FABRICATION OF BIEPITAXIAL YBazCuaO-i_, VOL. 36
a 800 A-thick YBa$usOr_y (YBCO) 1 ayer. Next, the CeOz/YBCO layer was lifted off by
using hydrochloric acid. Then, this substrate was annealed at 1100 ?C for 10 hours. To
examine the changes of the CeOz surface, both the surface morphologies of the annealed
and the unannealed Ce02 layers were taken by atomic force microscope (AFM) followed by
depositing a 2000 A-thick YBCO layer onto the annealed substrate. Moreover, the YBCO
thin film with a biepitaxial grain boundary was patterned photolithographically into a 5
pm-wide Josephson junction or a SQUID with the hole area of 20 x 40 pm2 and the junction
width of 5 pm. To investigate the microwave responses of the junction, a microwave was
guided to the junction by using a diode antenna. For checking the magnetic field modulated
voltage oscillation, the SQUID was mounted in a solenoid which provided a magnetic field
perpendicular to the plane of the SQUID. The traditional four-probe method was used for
electric measurements and the criterion voltage for the critical current was 1 JLV in this
work.
III. Results and discussion
In order to confirm the good quality of the YBa2CusOr_, (YBCO) thin film, the
temperature dependent resistances of YBCO thin film on the MgO substrate and that on
the unannealed CeO2 layer were measured, as shown in Fig. 1. The YBCO film on the
MgO substrate exhibits a superconducting behavior with T,,,,, = 84.2 K. However, the
YBCO film on the unannealed CeO2 layer shows a semiconducting resistive behavior with
a little drop at temperature around 82 K. This semiconducting resistive behavior may be
due to the following causes. First, the composition of YBCO is changed as it was sputtered
on the unannealed CeOa layer. Secondly, the grain of the YBCO is not planar. To check
the first point, powder x-ray diffraction pattern was detected for the unannealed sample
and shown in Fig. 2. It was observed that all the peaks of {O,O,n} planes for YBCO and
CeOz are clear and sharp. This implies that the compositions for YBCO and CeOz layers
are correct and hence the first cause is exclusive. Next, the surface profile of the YBCO
thin film on the unannealed CeO2 layer was probed by the atomic force microscope (AFM),
as shown in Fig. 3. It was found that many spiral grains appear on the surface of the
YBCO thin film, as indicated by arrows. The spiral grains result from the dislocation of
the layered structure in YBCO. Since the transport properties of YBCO are dominated
by its layered structure and become poor as the layered structure is deformed., Hence, the
spiral grains are responsible for the semiconducting resistive behavior of the YBCO thin
film on the unannealed CeOz layer.
To improve the quality of YBCO thin film on the Ce02 layer, the MgO substrate
covered with a CeOz layer was annealed at 1100 ?C for 10 hours under one atmosphere
of oxygen before depositing the YBCO thin film. The resistance versus temperature for
the YBCO thin film on the annealed CeO;! layer was shown in the inset of Fig. l(b).
It was observed that the YBCO thin film reveals a good superconducting behavior with
Tc,zero -- 87.2 K. Hence, the annealing process for the CeO2 layer is essential for improving
the quality of YBCO/Ce02 thin films. In order to investigate the changes of the CeO2
layers before and after being annealed, the surface images of the CeO2 layers were probed
by AFM and were shown in Fig. 4. Some surface features are listed in Table I. It was found
that the grain size becomes much larger, meanwhile, the CeO2 surface turns out rougher