THz帯SIS素子の開発の現状－ 1.5--2.0 GHz帯SISミキサの開発に向けて －野口卓、 山下 弘基、 酒井(電気通信大学)

剛Requirements for a heterodyne
receiver at 1-2 THz bandHEBOperation(center)frequency
Fractionalfrequency
bandwidth
IF frequency
bandwidth

SIS

SIS junctions to be used forheterodyne receivers at 1-2 THz band 1
Operating frequency and gap energy

Gap energy
(meV)

NbNbN

Nb3Ge

Nb

1.35

2.7 meV
1.30THzNbN

2.36

3.71 meV
1,80 THz

4,72 meV2.28 THz

Nb3Ge

3.46

4.81 meV2.33 THz

5.82 meV2.81 THz

7.92 meV3.35 THz

NgB2

~6.5

7.85 meV
3.80 THz

8.86 meV4.28 THz

9.96 meV4.82 THz

MgB213 meV
6.3 THzCurrent density of SIS junctions

SIS junctions to be used forheterodyne receivers at 1-2 THz band 2
Native barrier
O2

SCDeposited barrier

SC

N*SC

Oxide or Nitride

SC

Oxide or NitrideSCSC

Current density of SIS junctions

M. Kroug et al., to be published.
H. Ishida et al., Electronics and Communicationsin Japan, Part 2, Vol. 88, No. 12, 2005
Z. Wang et al., Appl. Phys. Lett. 70, 114 (1997).

Nb/Al接合のサブギャップ電流について

Niobium SIS Junctions with Al Cap Layer previously reported:SIS Junctions Based on AlNx Barrier- jc = 10 – 45 kA/cm2- sharp non-linearityand low leakage with q > 20Nb

Al cap layer 3-4 nm
barrier ~1nmNb

- several devices successfully tested
in the 400 – 500 GHz range(including a jc ~ 45 kA/cm2 mixer)
 same approach recently used for:AlOx barrier based SIS junction

SIS Junctions Based on AlOx Barrierjc ~ 25 kA/cm2- junction quality comparable forcurrent density up to 25 kA/cm2 j ~ 0.75 m- fabrication not optimized yet we have a promising additional option
for fabricating high-jc SIS mixers

q = 20Al over layer
6-8 nmUltra high current density SIS junctionNb

Al cap layer 3-4 nm
AlN barrier ~1nm

μNb

IV characteristics of three junctions, nominal
diameter j = 1.0, 0.9 and 0.8 m, made fromthe layer stack Nb/Al,AlNx/Al/Nb.

Al over layer 6-8 nm

6-8 nmHigh Critical Current Density (Jc)
SIS Junction Device Development
200
Blue: Band 8 production (13 kA/cm 2)
Red: High Jc mixer (24 kA/cm2)

Trx, (DSB) [K]150

100500400

420440LO Frequency [GHz]Band 7Band 8275 – 370 GHz
385 – 500 GHz

460480500本研究の目指す技術開発
Nb3Ge/NbOx/Al/Nb接合の新規性、利点
・Nb3Ge/NbOx/Al/Nb接合自体が新規な構造
・Pbに代えて、化学的、機械的に安定なNbを利用→SIS素子の安定性、信頼性の向上→実用的なSIS素子へ
・Alバッファ層の活用→酸化層のNbとの反応による劣化を低減、防止→リーク電流の低減→超高電流密度接合の実現
・同一真空中で、Nb3Ge/NbOx/Al/Nb接合構造を作成→素子製造の再現性、安定性の向上
Nb
NbOx

Al
Nb3Ge
X’tal Al2O3 (sapphire) substrate

Simulated RF response of a Nb3Ge/barrier/Nb junction

Nb3Ge/SiO2/Pb junction

M. Muck et al., IEEE Trans. Magn.,Vol. 23(2), 1493 – 1496, 1987.

Note:Pb was etched by FeCl3 solution,
whereas Nb3Ge was etched reactively
in an SF6 plasma.

I-V curves of Nb-based SIS junctions

R. F. Broom et al., IEEE Trans.
Electron Dev., ED-27, 1998, (1980).

High VG
material
G. Hawkins and J. Clarke.
J. Appl. Phys., 47, 1616 (1976)

Nb3Ge/NbOx/Pb junction

K. Tanabe and O. Michikami, J. Appl. Phys.,vol.58(9), 3519-3528, 1985.

Substrate: (1102) Sapphire (R plane)Nb3Ge preparation:
DC magnetron sputtering
Target: Arc melted Nb0.742Ge0.258
Subst. Temp.: ~700 ℃
Sputt. Rate: ~20 nm/min.
Thickness: 300-400 nm
Tc : 21.3 K(Breaking vacuum)
Base electrode patterning: Chemical etching(Breaking vacuum)
Base electrode cleaning: Ar + 9.7vol%CF4Oxidation: Plasma oxidation, Ar + 8.1vol%O2
4 Pa, 60 V, 1-30 min. (typcal)
Pb deposition:
Thermal deposition, 400-600 nm

・ 物理的、化学的に安定・ ドライエッチングが可能 → 写真製版による微細加工が可能

Suppression of leakage current
NbOx(Semi-cond.)

NbAl

Al2O3AlNb or Nb3Ge

Nb or Nb3Ge
NbAlAlNb or Nb3Ge

従来技術の拡張
Nb3Ge/NbOx/Pb → Nb3Ge/NbOx/Al/Nb

K. Tanabe and O. Michikami, J. Appl. Phys.,vol.58(9), 3519-3528, 1985.

Substrate: (1102) Sapphire (R plane)
Nb3Ge preparation:
DC magnetron sputtering
Target: Arc melted Nb0.742Ge0.258
Subst. Temp.: ~700 ℃
Sputt. Rate: ~20 nm/min.
Thickness: 300-400 nm
Tc : 21.3 K(Breaking vacuum)
Base electrode patterning: Chemical etching(Breaking vacuum)
Base electrode cleaning: Ar + 9.7vol%CF4Oxidation: Plasma oxidation, Ar + 8.1vol%O2
4 Pa, 60 V, 1-30 min. (typcal)
Pb deposition:
Thermal deposition, 400-600 nm
Al/Nb deposition:
E-beam deposition, 400-600 nm

Fabricaton of a SIS junction with a Nb3Ge base electrodeNbNbOxAlNb3Ge
X’tal Al2O3 (sapphire) substrate

Procedure of the growth of the quadruple-layers （同一真空中で真空を破らずに実施）
1

Substrate temperature elevation

Up to 950 ℃

Sputter chamber

2Nb3Ge sputter

Φ2 inch target, confocal config.

Sputter chamber

3

Substrate temperature cool down

Down to room temperature

Sputter chamber

4

Plasma oxidation (NbOx)

10%O2/Ar

Load locked chamber

5Al deposition< 10 nm thick

Sputter chamber
or evaporation chamber

6Nb deposition

100 nmSputter chamber

MBE/スパッタ成膜システム構成図
蒸着成膜室
R-HEED
基板加熱機構スパッタ成膜室
基板交換室
基板加熱機構( < 1,000 ℃)

プラズマクリーニング
プラズマ酸化(接続済、未検証)(ES-350)
P < 1x10-6 Pa

P < 1x10-5 Pa

P < 1x10-5 Pa

Ar O2
(未接続)

E-ガン ポート（３連）: 1基
(3 kW, Au, Al, Ta)
K-セル: ３ポート
実装：1基(Al用)

DCスパッタガン:
４ポート
実装：２基 ( Nb, Al)

プロセス基板2 inch サファイヤ、シリコン
基板加熱（蒸着成膜室、スパッタ成膜室)
1000℃ Max、(800℃運用)

スパッタ装置 ES-350改
ロードロックチャンバ

スパッタチャンバ

加熱ステージマニピュレータ

Nb3Ge薄膜の試作

Fabricated and measured by H. Yamashita

Sputtering apparatus:Ar pressure = 0.4Pa, Shutter: open, 12 nm/min: Ar pressure = 0.8Pa, Shutter: open, 7 nm/min: Ar pressure = 0.8Pa, Shutter: close, 2 nm/min: No transition

Nb3Ge film on 35-mmΦ sapphireFuture work・ 高Tc Nb3Ge膜の作成条件の確定
・ Nb3Ge/Oxide/Al/Nb SIS接合の作成
oxide: Al2O3 or Nb2O5

Al2O3orNb2O5NbAlNb or Nb3Ge