Scholarly article on topic 'Fabrication and evaluation of hetero-epitaxial multilayer films of Nb/AlN/Nb/NbN for SIS junction'

Fabrication and evaluation of hetero-epitaxial multilayer films of Nb/AlN/Nb/NbN for SIS junction Academic research paper on "Materials engineering"

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{"magnetron sputtering" / niobium / "epitaxial growth" / "residual resistivity ratio" / Nb/AlN/Nb}

Abstract of research paper on Materials engineering, author of scientific article — M. Konno, T. Sawada, M. Murata, A. Kawakami, A. Saito, et al.

Abstract We investigated the fabrication of single-phase Nb films and hetero-epitaxial multi-layered films of Nb/AlN/Nb for SIS junctions. The Nb and multi-layered Nb/AlN/Nb films were deposited on NbN buffered MgO(100) single-crystal substrates at 753K by using a dc magnetron sputtering system. We observed that the Nb films grow to Nb(200) orientation by measuring a standard 2θ/θ X-ray diffraction. The φ-scan diffraction peaks of Nb(200) were rotated by 45 degrees in-plane orientation for those of both NbN(220) and MgO(220). We found that NbN buffer layer with the thickness of a few nm was effective to enhance the Nb(200) orientation. The best thickness for a NbN buffer layer to grow epitaxial of Nb film was approximately 3nm. Also, we examined dependence of degree of Nb(200) orientation and surface roughness(Ra) on multi-layered films. As a result, we found that the multi-layered films had flat surface interfaces for SIS junctions almost equivalent to single-layered films.

Academic research paper on topic "Fabrication and evaluation of hetero-epitaxial multilayer films of Nb/AlN/Nb/NbN for SIS junction"

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Physics Procedia 27 (2012) 304 - 307

ISS2011

Fabrication and evaluation of hetero-epitaxial multilayer films of Nb/AlN/Nb/NbN for SIS junction

M. Konnoa, T. Sawadaa, M. Murataa, A. Kawakamib, A. Saitoa*, K. Nakajimaa, S. Ohshimaa

a Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan b KARC, National Institude of Information and Communication Technology, 588-2 Iwaoka, Iwaoka-cho, Nishi-ku, Kobe 651-2492, Japan

Abstract

We investigated the fabrication of single-phase Nb films and hetero-epitaxial multi-layered films of Nb/AlN/Nb for SIS junctions. The Nb and multi-layered Nb/AlN/Nb films were deposited on NbN buffered Mg0(100) single-crystal substrates at 753 K by using a dc magnetron sputtering system. We observed that the Nb films grow to Nb(200) orientation by measuring a standard 20/0 X-ray diffraction. The 9-scan diffraction peaks of Nb(200) were rotated by 45 degrees in-plane orientation for those of both NbN(220) and Mg0(220). We found that NbN buffer layer with the thickness of a few nm was effective to enhance the Nb(200) orientation. The best thickness for a NbN buffer layer to grow epitaxial of Nb film was approximately 3 nm. Also, we examined dependence of degree of Nb(200) orientation and surface roughness(Ra) on multi-layered films. As a result, we found that the multi-layered films had flat surface interfaces for SIS junctions almost equivalent to single-layered films.

© 20121 Published by Elsevier B.V. Selection and/or peer-review under responsibility ofISS Program Committee keywords: magnetron sputtering; niobium; epitaxial growth; residual resistivity ratio; Nb/AlN/Nb

1. Introduction

Detector technologies using a superconductor-insulator-superconductor (SIS) with all niobium (Nb) electrodes are gradually approaching the quantum noise limit in the millimeter and sub-millimeter wave. A detector is desired the reduction of the sub-gap leakage current to the limit indicated in the BOS theory. The sub-gap leakage current in the SIS mixer reduces exponentially as temperature goes down, but the reduction is saturated below a given temperature [1-3]. Improving quality of Nb films is expected to obtain a low sub-gap leakage current at low temperature [2]. There is a report of growth single-crystal of Nb films on sapphire substrates [4]. However, because there have been few experiments on SIS junctions using hetero-epitaxial multilayer films, the experimental comparison between poly-crystal and single-crystal of multi-layered films have not been experimentally compared. In this work, we fabricated the single-phase Nb thin films and multi-layered films of Nb/AlN/Nb using NbN buffered MgO substrates. A high substrate temperature may be necessary to obtain epitaxial Nb. During the deposition of the multi-layered films, a high substrate temperature will induce impurity diffusion and interaction at the interface in the multi-layered films using low melting point materials [5]. Therefore aluminium nitride (AlN) was chosen as the insulator, because the very hard physically, the strong bond between the atoms, thermal, and chemical stability. Also, we investigated the flatness of the interface in the multi-layered films and the condition in which the multi-layered films are epitaxially grown.

* Corresponding author. Tel.: +81-238-26-8294 ; fax: +81-238-26-8294 . E-mail address: atsu@yz.yamagata-u.ac.jp .

1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of ISS Program Committee doi:10.1016/j.phpro.2012.03.471

2. Experimental procedure

2.1. Deposition method of Nb/NbN and multilayered Nb/AlN/Nb/NbN films

The Nb/NbN and multilayered Nb/AlN/Nb/NbN films were deposited on Mg0(100) substrates by using a dc magnetron sputtering system. A 10 mm*10 mm*0.5 mm Mg0(100) single crystal substrate was intentionally heated during the deposition by using a lamp heater at substrate temperature (Ts) of 753 K. To enhance the epitaxial growth of the Nb films, we used the NbN buffer layer on the MgO substrates. The NbN buffer layer, the base-Nb electrode, the AlN layer, and the top-Nb electrode had thicknesses of 3 nm, 100 nm, 1-4 nm, and 50 nm, respectively. The films were sputtered in a chamber with a base pressure less than 1.35* 10-6 Torr. The targets of niobium (075 mm, 99.99 %) and aluminum (050 mm, 99.99 %) were used for the sputtering. The other sputtering conditions are listed in Table 1. The NbN buffer films were fabricated by reactive dc magnetron sputtering of niobium in an Ar-N2 mixture [5].

Table 1. Sputtering conditions of Nb, NbN, and AlN films

Material Discharge power (W) Ar flow (sccm) N2 flow (sccm) Total pressure (mTorr)

NbN 102 2.12 070 2.0

Nb 385 2.42 0 2.0

AlN 100 0 6.0 2.3

2.2. Evaluation method

The crystal structures of the Nb films and multilayered Nb/AlN/Nb/NbN films were studied using standard 20/0 X-ray diffraction (XRD) measurements with Cu-Ka radiation. The resistances vs. temperature (R-T) curves for the films were measured by the standard four-probe method. We measured the surface morphology of these films by the dynamic force microscopy (DFM) in order to investigate the microstructures of the films.

3. Result and discussion

3.1. Crystal structure and electrical properties of Nb films

Fig. 1 (a) shows typical XRD patterns of Nb films deposited on NbN buffer films with the thicknesses of 0 and 10 nm. The thickness of all of the Nb films was approximately 433 nm. The Nb(433 nm) film deposited on the MgO substrate showed both Nb(110) and Nb(200) diffraction peaks as shown in the data in Fig. 1 (a). By contrast, only a strong Nb(200) diffraction peak of Nb(433 nm) film deposited on NbN(10 nm) buffered MgO substrate was observed. Fig. 1 (b) shows typical XRD qi-scan diffraction patterns of Nb film deposited on NbN buffer film with thicknesses of 10 nm. We used the Nb(110), NbN(220) and Mg0(220) ^-scan diffraction peaks. We found that the NbN(220) and Mg0(220) qi-scan diffraction peaks appeared at the same angles, at intervals of 90 degree. Also, qi-scan diffraction peaks of Nb(200) are rotated 45 degree in-plan orientation for qi-scan diffraction peaks of NbN(220) and Mg0(220). The lattice constants of Nb, NbN, and Mg0 are 0.330 nm, 0.439 nm, and 0.421 nm, respectively. The lattice mismatches were 4.2% for cube-on cube NbN/Mg0 and -24.8% for cube-on cube Nb/Mg0. However the lattice mismatch was 6.3% for NbN/Mg0, when Nb(200) was rotated 45 degree in-plan orientation. The lattice parameter calculated from typical XRD pattern was 0.440 nm for NbN(200) and larger than the lattice parameter for bulk NbN, 0.439 nm. As a result, we found it important to facilitate hetero-epitaxial growth of Nb films that may be used as a NbN buffer layer on a Mg0(100) substrate.

Nb(433 nm)/MgO

Nb(433 nm)/NbN(10 nm)/MgO

I I I I I I I I I I I I I I I I I

50" 1 55

29 [degree]

I I I I I I I I I 6

(b) 30

: (d) NbN(220) ^-scan diffraction pattern

Nb(110) ^-scan diffraction pattern

I I I I I I I I ill I I I I I I I iti I I I I I I I ill I I I

J.l.lJ-LJ-1-LdJ-J.l.ldl-l.L

tluddl

- (e) Mg0(220) ^-scan diffraction pattern

......

I I I I 11 I I I I I I I 11 I I I I I I I 11 I I I I I I I

0 50 100

150 200 250 ^[degree]

300 350

Fig. 1. (a) XRD patterns of Nb(433 nm)/MgO and Nb(433 nm)/NbN(10 nm)/MgO; (b) 9-scan diffraction patterns of Nb(433 nm)/NbN(10 nm)/MgO: (c) Nb films, (d) NbN films, and (e) MgO substrate.

(a)g fi 100 80

fit © «3 « fi £ i ^ s« © 60 40

.o N 0

2 4 6 8 10 12 NbN film thickness [nm]

2 4 6 8 10 NbN film thickness [nm]

Fig. 2. (a) Dependence of degree of Nb (200) orientation on NbN film thickness; (b) Dependence of Tc and RRR on NbN film thickness.

To investigate the 200 orientation of the Nb films, we defined the degree of Nb(200) orientation (D200) as below:

D200[%] =——— x 100 (1)

1200 + 1110

where /200 and /110 are the intensities of Nb(200) and Nb(110), respectively. Fig. 2 (a) shows the dependence of the D200 of the Nb films on the thickness of the NbN buffer films. The solid line shows an eye-guide line for connecting these experimental data. The thickness of all Nb films to which an electrode of SIS junctions was applied was 100 nm. The thicknesses of these NbN buffer films were varied from 0 to 10 nm. The D200 values were increased with the thicknesses of NbN varied from 2 to 5 nm and decreased at the thicknesses of 10 nm. Also, when the NbN buffer film was 5 nm thick, it showed higher orientation. These results indicate that NbN buffer films with the thickness of a few nm were effective to obtain the Nb(200) orientation which means the epitaxial growth of the Nb films on the NbN buffered MgO substrates. In order to investigate the cause of these results, the 29/9 measurements of these samples were carried out. Though no clear XRD NbN(200) peaks were observed for NbN with thickness 5 nm, we speculate that the D200 values vary by small lattice constant changes in these NbN buffer layers. Fig. 2 (b) shows the dependence of critical temperature Tc and residual resistivity ratio (RRR) on the thickness of NbN buffer films. The solid line is an eye-guide line for connecting the data. To investigate the quality of the Nb films, we defined the RRR value as RRR=R300k/Ri0k, where R300 K and R10 K are the resistances of temperatures at 300 K and 10 K, respectively. When the NbN buffer films were 10 nm thick, Tc was 12 K. This was influenced by superconducting properties of NbN, when the NbN buffer films were 10 nm thick [6]. On the other hand, when the NbN buffer films were 3 nm thick, RRR value showed the higher value of 6.44. Therefore, we could obtain the higher RRR value for Nb/NbN films deposited on the 3 nm thick NbN buffer films.

3.2. Surface morphology and crystal structure of multi-layered Nb/AlN/Nb/NbN films

Fig. 3 (a) shows XRD patterns of Nb/AlN/Nb/NbN multi-layered films. These thicknesses of AlN films are from 0 to 4 nm. The base-Nb electrode, the top-Nb electrode, and NbN buffer films had thicknesses of approximately 100 nm, 50 nm, and 3 nm, respectively. All Nb/AlN/Nb/NbN multi-layered films showed Nb(200) diffraction peaks, but Nb single layer with thickness of 150 nm shows higher Nb(200) diffraction peak. Therefore, we investigated dependence of D200 on film thickness of AlN insulator. Fig. 3 (b) shows dependence of D200 on the thickness of AlN films. The solid line shows an eye-guide line. The thicknesses of AlN films were varied from 0 to 4 nm. The Nb/AlN/Nb/NbN in all AlN film thickness shows the Nb (200) orientation. The D200 of all AlN film thicknesses were greater than 80 %, but were more than 95 % for single Nb films. Therefore, the grain and in-plan orientation of the top-Nb electrode layer of the multilayer might be not perfectly oriented.

Nb(110) 'o ^ Mg0(200) O: MgO substrate 0) 0 §1 AlN film thickness . A 0 nm

A 1 nm

A 2 nm

A 3 nm

J * A 4 nm

1 II 1 | Il II | Il II | 1 II 1 | 1 1 II | II 1 1 II 1 1

35 40 45

50 55 20 [degree]

100 80 60 40 20 0

60 65 70

12 3 4 AlN film thickness [nm]

Fig. 3. (a) X-ray diffraction patterns of Nb/AlN/Nb/NbN films; (b) Dependence of degree of Nb (200) orientation on AlN film thickness.

30 H 15

.¡^ooO

js 500 0

0 50 100 150 200 250 300 350 ^ [degree]

Fig. 4. 9-scan diffraction patterns of Nb (200): (a) Nb/NbN (150/3 nm)films and (b) Nb/AlN/Nb/NbN (50/1/100/3 nm)films.

Table 2. Characteristics for surface roughness and crystal structure of Nb/AlN/Nb/NbN films

AlN film thickness [nm] 0 1 2 3 4

Ra [nm] FWHM of Nb(200) diffraction peaks [deg.] FWHM of Nb(110) 9-scan diffraction peaks [deg.] 0.73 1.31 1.50 0.71 1.20 1.70 0.74 1.08 1.50 0.73 1.18 1.60 0.70 1.20 1.65

Fig. 4 shows typical XRD qi-scan diffraction patterns of Nb(150 nm)/NbN(3 nm) single-layered and Nb(50 nm)/AlN(1 nm)/Nb(l00 nm)/NbN(3 nm) multi-layered films. We found that the Nb(110) ^-scan diffraction peaks of single-layered and multi-layered films appeared at the same angles, at intervals of 90 degree. The result of Fig. 3 and 4 indicated that the diffraction peak intensity of Nb(200) for multi-layered was smaller than that of single-layered. Therefore the crystal structure of top-Nb electrode of multi-layered films had poor orientation or fine crystal. Table 2 shows characteristics for surface roughness and crystal structure of Nb/AlN/Nb/NbN multi-layered films. Surface roughness multi-layered films were obtained flat compare with the single-layered films. This result could be expected that the Ra at the interfaces between the top-Nb electrode layer and the AlN layer of Nb/AlN/Nb/NbN multi-layered films were less than 0.74 nm. Because of roughness at interfaces of multi-layers might be smaller than the surface roughness. We have to observe the interfaces of the multi-layer employed the TEM and use a tunnel spectroscopy in future. The multi-layered films had almost the same crystal structure as the single-layered films. However, this result means that in-plane orientation of the base-Nb electrode layer of Nb/AlN/Nb/NbN multi-layered films was observed. Therefore, we will think about estimation by the reflection high energy electron diffraction (RHEED) in order to investigate the orientation of the top-Nb electrode of the multi-layered films.

4. Conclusion

We developed a method for examining epitaxial growth techniques of Nb films and characteristics of surface roughness and crystal structure of multi-layered Nb/AlN/Nb/NbN films by using the reactive dc magnetron sputtering. The varying thicknesses of NbN buffer films had effective to obtain the Nb(200) orientation and helped the epitaxial growth of the Nb films on the NbN buffered MgO substrates. The qi-scan diffraction peak positions of Nb(200) were shifted by 45 degrees compared with those of both NbN(220) and Mg0(220). The interfaces of multi-layers might be smaller than the surface roughness. The top-Nb electrode had poor orientation or fine crystal.

Acknowledgements

This work was supported by Grant-in-Aid for Young Scientists (C) ( 22560317) . A part of this work was partly carried out in the clean room of Yamagata University.

References

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