1635 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 5 , NO. 2, JUNE 1995

Pulsed Laser Deposition of Y B U ~ C U ~ ~ ~ - ~ / B ~ T ~ ~ ~ / Y B ~ ~ C U ~ O ~ - ~ Multilayer Structure on Si(1OO) Substrates

J. Luo, Q. Li, D. B. Fennel, S. Liu, W. D. Hamblen AFR, Inc., 87 Church Street, East Hartford, CT 06108

X. D. Wu Los Alamos National Laboratory, Los Alamos, NM 87545

Abstract-we have studied materials issues rele- vant to the applications of YBCO/BTO/YBCO mul- tilayer structures, including multilayer HTS circuits and crossovers, nonvolatile memories, and on-chip en- ergy storage for Si solar cells. Due to the good lattice match between BTO and YBCO, epitaxial YBCO/BTO multilayer device structures are possible. We have deposited YBCO/BTO/YBCO capacitors onto YSZ buffered Si substrates by using pulsed laser deposition technique. All four oxide layers are grown in situ without breaking the vacuum, and without low- ering the substrate temperature to below 60OoC. We have achieved sharp resistive transitions for both the top and the bottom YBCO layers with onset at 90 K, zero resistance at 89 K for the bottom layer and 88 K for the top layer. X-ray diffraction data indicate that all four deposited oxide layers have their c-axis per- pendicular to the substrate, and have less that 10% in- plane misorientation. The BTO layer shows a dielec- tric constant of 210, a leakage constant of 5 x 1 0 - 7 ~ / c m 2 at 10 Volts, and a remanent polarization of 26pClcm'.

I . INTRODUCTION

Since the discovery of high temperature superconduc- tivity, Y B u ~ C U O . ? - - ~ (YBCO) has been one of the most studied high temperature superconducting materials due to its exceptional superconductivity properties. In its nor- mal state, YBCO is metallic with a low thin film resistiv- ity. Furthermore, YBCO has a similar perovskite crystal structure and a small lattice mismatch with ferroelectric BuTiOs (BTO), making it a very attractive candidate for electrodes in epitaxial ferroelectric capacitors [l]. Ferro- electric thin film capacitors are the subject of considerable research interest because of their potential applications in nonvolatile memories, electro-optic switches, pyroelectric detectors, and on-chip energy storage for Si solar cells [a ] . When grown on Pt bottom electrodes, ferroelectric films are amorphous or polycrystaline, and are susceptible to chemical and charge segregation which leads to aging and fatigue. The advantage of using oxide electrodes is the

Manuscript received October 15, 1994. This work was supported in part by the U.S. Department of En-

ergy Grant No. DE-FG05-93ER81591.

similar chemistry between the electrodes and the ferro- electric films. Epitaxial ferroelectric films grown on lat- ticed matched substrates have shown improved structural quality and enhanced capacitor performance endurance [3]. In this work, we examine the growth of highly ori- ented ferroelectric BTO films sandwiched between a pair of lattice matched YBCO electrodes. This architecture can also be used as a basic building block for stacked mul- tilayer ferroelectric capacitors to achieve high capacitance per unit area.

11. EXPERIMENT

We have deposited YBCO/BTO/YBCO/YSZ/Si struc- tures in sztu by using pulsed laser deposition technique. Si (100) wafers are the substrate of our choice in this research because the resulting thin capacitor structures can be more readily integrated with standard Si micro- electronic devices. The yttria-stabilized Zr02 (YSZ) buffer layer prevents the interfacial interaction between Si and YBCO at the growth temperature and to pro- vide a suitable epitaxial template between these two ma- terials [4]. In an ideal epitaxial configuration of the YBCO/BTO/YBCO/YSZ/Si structure, YSZ grows in a cube-on-cube orientation on Si (100) surfaces, i.e., with a-axis and b-axis parallel to those of Si substrate. The bottom electrode YBCO grows with its a-axis parallel to the YSZ < 110 >, i.e., with a 45' rotation in the plane of the interface. The BTO dielectric layer and YBCO top electrode grow in a cube-on-cube epitaxy configuration onto the previous layers. Fig. 1 shows the schematics of the device structure and the epitaxial relations between the layers. Recent studies on the epitaxial interfaces be- tween YBCO/BTO heterostructures revealed that YBCO grows epitaxially without any observable interfacial layer on BTO surface [l]. Multilayer capacitors of good quality crystalline and electric properties are expected.

Our pulsed laser deposition system is based on a Con- tinuum Corp. Nd:YAG laser, model NY-61-10, which pro- vides UV light with 60 mJ/pulse at 266 nm. The deposi- tion rate and the film thickness are monitored by a mod- ified Bomem MB-155 Fourier-transform infrared (FTIR) spectrometer [5]. In our work, We prepare single side polished Si wafers by following a standard semiconductor degreasing procedure and a spin-etch for H-termination

1051-8223/95$04.00 0 1095 IEEE

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I I I I i I 1 I I

Schematic diagram of a YBCO/BTO/YBCO capacitor

0 0

Si(100) ed *" Electrode: cuprate superconductor YBCO

Dieketrk barium titanate (BTO) YSZ coated Si(OO1)

Fig. 1. Schematic di,agram of a YBCO/BTO/YBCO/YSZ four-1aye.r structure on Si [OOl] substrate and the epitaxial orientations betwern the different layers. There is a 45O rotation at the interface between YBCO/YSZ. All other interfaces have cubic on cubic epitaxial rela- tion.

process. The Si substrate is then immediately introduced into the deposition chamber, which is evacuated to a base pressure of Torr. The sintered targets of YBCO, BTO, YSZ are mounted onto a three-target translating finger, thus enabling in situ deposition of the multilayer structure. The deposition is carried out in oxygen pres- sure of 0.4 mTorr to 200 mTorr, and the deposition rate is between 0.5 to 5 A/sec. We deposited the layers in the sequence without breaking the vacuum, and without lowering the substrate temperature below 600°C. Before the deposition of each subsequent layers, we cover a larger area of the substrate by using a shutter mounted inside the deposition chamber. Thus each of the four layers have a portion of the surface exposed to allow external electri- cal contacts.

For meaSur ing electrical properties of YBCO/BTO/YBCO capacitors, we patterned the top YBCO electrode into a round disk of diameter of 1 mm through wet chemical etching. The ferroelectric properties of BTO films were investigated by means of a computer controlled Sawyer-Tower circuit. Current- voltage studies were carried by using a Fluke model 8842 electrometer and a model 230 Keithley programmable voltage source. Capacitance-voltage (C-V) measurement were carried out by using a PAR model 5210 lock-in amplifier operating at 100 kHz.

111. RESULTS

The details of deposition and structure analysis of the bottom electrode YBCO on YSZ buffered Si substrates are reported elsewhere [6]. The YSZ buffer films were charac- terized by x-ray diffraction on a four circle diffractometer, which shows in-plane alignment and an w-rocking curve widths below 0.6". Transmission electron microscopy

104

E i * 103

n

+a Y

m .- 2 z 102 U

101

100 10 20 30 40 50 60 70 80 90

29 (deg.) Fig. 2 . X-ray diffraction 0 - 20 pattern from four-layer structure YBCO/BTO/YBCO/YSZ grown on Si [OOl] substrate. The un- marked peaks are YBCO 001 diffraction, which are equally spaced. This diffraction pattern shows the presence of only 001 lines for YBCO and BTO alongwith those of YSZ and Si. Therefore, the c-axes of all the layers are perpendicular to the substrate.

(TEM) revealed sharp YBCO-YSZ and YSZ-Si interfaces. The YBCO bottom electrodes typically have transition temperature T, - 89 K and superconducting current den- sity J , - 2 x 106A/cm2 at 77 K.

In this paper, we present results from a YBCO/BTO/YBCO/YSZ/Si structure, in which the top and the bottom YBCO electrodes are both 600 Aand the BTO layer is 1500 A. The thickness of YBCO electrodes is carefully chosen in order t o avoided degradation caused by cracks developed for films thicker that 600 A. Table I lists the deposition conditions for this sample. We have studied the structural properties of the device through x-ray analysis. Fig. 2 shows x-ray diffraction 0 - 20 spec- trum. The diffraction peaks are identified as U01 peaks for YBCO and BTO, in addition to the ( 0 0 2 ) ~ s ~ and (004)si peaks. This indicates that all of the deposited layers have their c-axis perpendicular t o the substrate.

TABLE I GROWTH CONDITIONS OF A

YBCO/BTO/YBCO/YSZ/Si CAPACITOR

# Layer Thickness(A) Temperature ("C) OzPressure (mTorr)

1) YSZ 500 800 0.4 2) YBCO 600 790 200 3) BTO 1500 750 200 4) YBCO 600 780 200

We have studied in-plane texturing of the multilay- ers by taking x-ray diffraction +scans of YBCO{lOS}, BTO{101}, YSZ(202) reflections with the 4 axis normal to the plane of the substrate. Fig. 3 shows x- ray diffrac- tion 4 scans from the sample listed in table I. The zero of 4 angle is determined from the peaks of Si substrate, which

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occur at 4 = 0",90", 180°, and 270". Two types of peaks in the spectrum are discernible: one set at 4 = 0",90", etc.; and the other set at the odd multiples of 45'. The 4 = 0" set of peaks indicate a c-axis oriented film with a-axis parallel to Si < 100 >, i.e., cubic-on-cubic epitaxy or 4 = 0" material with respect t o the Si substrate. The 4 = 45" set of peaks originate from c-axis films with a- axis parallel to Si < 110 >. Our results shows YSZ grows cube-on-cube on Si, and YBCO has the expected 45" rota- tion with respect t o that of YSZ.-BTO spectrum contains a mixture of 4 = O", and 4 = 45" peaks. The misori- entated (or, the unrotated) component at 4 = 0" has an intensity of 10% of the epitaxial component at 4 = 45". It is interesting to notice that the YBCO spectrum contains less that 1% of the misoriented component at 4 = 0". The YBCO spectrum is from the combined contributions from the top and the bottom layer YBCO and the bot- tom YBCO is 100% epitaxial, thus, the 1% misorientated component is all from the top YBCO layer. This trans- lates into a 2% misorientated fraction in the top YBCO layer. This is much smaller than in the BTO layer. It seems that the YBCO layer has recovered partially from the misorientation of the BTO layer below. This can assist in maintaining epitaxial quality when additional layers are grown onto the top YBCO.

In Fig. 4 we show an optical Nomarski micrograph of tlie YBCO/BTO/YBCO capacitor on YSZ/Si substrate. The mosaic block structure on the top YBCO electrode surface is familiar in epitaxial growth of YBCO [7]. It may be due to crystal growth from different nucleation, domain twinning in the films, or due to the lattice mis- match between the films and substrates. Due to the in- creased thermal expansion mismatch, the YBCO films on Si substrates are under higher tensile stress and tend to crack the substrate cools from the deposition temperature to room temperature. We have observed microcracks in thick (> 700A) YBCO films on YSZ buffered Si substrate due to excessive tensile stress.

We have also examined the electrical properties of the device. Fig. 5 shows the 4-point resistivity as a function of temperature for the top and the bottom YBCO layer. We have an onset at 88 K for the bottom layer, and 87 K for the top layer. Both layers have sharp resistive tran- sitions. The good superconducting properties of the top YBCO layer suggests that no signification degradation in film quality when it is deposited on a BTO layer. The MIM capacitance of YBCO/BTO/YBCO is voltage inde- pendent up to the maximum applied DC bias of 30 volts. This indicates good insulating properties of the BTO film, which is confirmed by a high DC resistivity of 10l1 R-cm measured across the device. The dielectric constant of the BTO film is calculated to be 210 from the measured capacitance value and the dimensions of the device. The ferroelectric properties of the BTO film was examined by measuring polarization-hysteresis behavior. A E-P hys-

l O 9 t I I I I I 108

107 n

0

0 90 180 270 Phi (deg.)

Fig. 3. X-ray difiaction 4 scans of the YBC0{103}, BTO{101}, and YSZ(202) family of reflections for a YBCO/BTO/YBCO/YSZ four- layer structure on Si [OOl] substrate. The diffraction peaks of Si(100) occur at q5 = Oo, 90°, 180°, 270O. YSZ spectrum shows 6 = 0" peaks, indicating cubic on cubic growth on Si substrate. YBCO spectrum shows a majority of q5 = 45" peaks with a small amount (less than 1%) unrotated component a t q5 = Oo. BTO spectrum has 90% of 4 = 4 5 O peaks and 10% of unrotated component q5 = 0. This data indicates that the a- and b- axes of all the layers are also aligned.

Fig. 4. Nomarski optical micrograph of the YBCO/BTO/YBCO/YSZ/Si device discussed in the text. The mosaic block structure is familiar in the epitaxial growth of oxide films

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REFERENCES

Temperature (K)

Fig. 5. Resistive transitions from the top YBCO layer and the bottom YBCO layer of a four-layer epitaxial structure YBCO/BTO/YBCO/YSZ on a Si substrate. Both layers have a sharp resistive transition. The good superconducting properties of the top YBCO layer suggests that no there is no signification degra- dation in film quality when i t is deposited on a BTO layer.

teresis loop was observed by using the Sawyer and Tower circuit at a frequency of 1 kHz. The hysteretic remanent polarization is determined to be Pr = 26 pC/cm2.

IV. SUMMERY

In summery, we have demonstrated the growth of YBCO/BTO/YBCO capacitors on YSZ buffered Si sub- strates by using pulsed laser deposition technique. All of the oxide layers were grown in situ without breaking the vacuum and without lowering the substrate temper- ature to below 600°C. We have achieved a highly tex- tured YBCO/BTO/YBCO capacitor which has not only c-axis orientation in all the layers, but also a high de- gree of in-plane orientation as well. A fully epitaxial YBCO/BTO/YBCO capacitor is expected by eliminating the 10 % in-plane misorientation of BTO layers through further optimization of the deposition temperature and oxygen pressure. Our results have demonstrated that YBCO can be used as metallic electrodes in epitaxial ca- pacitors with stacked YBCO/BTO multilayer structure.

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