Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Tunneling through region of reduced conductivity
Reexamination Certificate
1999-05-19
2002-02-19
Wilson, Allan R. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Tunneling through region of reduced conductivity
C257S031000, C257S033000, C505S190000, C505S234000
Reexamination Certificate
active
06348699
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The invention relates to superconducting devices, corresponding technologies and application fields, and more specifically to a novel generator and detector of sub-millimeter electromagnetic radiation, and its multiple applications.
(b) Description of the Related Art
1. Arrays of Artificial Junctions
The first realization of potential usefulness of Josephson junctions as tunable microwave sources and detectors can be traced back to the earliest works of B. Josephson and S. Shapiro. It was also understood very early that a single Josephson junction emits with too little power and too broad linewidth to be useful as a practical microwave source. These deficiencies can be removed by using arrays of Josephson junctions [Jain et al. 1984; Bindslev Hansen and Lindelof 1984; Lukens 1990]. If the coupling between the junctions is strong enough, phase locking may occur between them; in this cases, the array emits coherent radiation [Lukens 1990; Konopka 1994]. Possible coupling mechanisms and coupling strengths have been analyzed in detail [Jain et al. 1984; Lukens 1990]. It has been understood that the linewidth of the electromagnetic radiation emitted from an array of Josephson junctions decreases as the number of junctions within the array is increased, and can become very narrow in large arrays [Lukens 1990; Wiesenfeld et al. 1994; Konopka 1994].
Power of the emitted radiation also increases with the number of junctions in the array, and in large arrays it can become large enough (P≧1 mW) for many practical applications [Bindslev Hansen and Lindelof 1984; Jain et al. 1984; Konopka et al. 1994; Wiesenfeld et al. 1994]. It is important here that a good impedance matching is achieved with the load, because in the opposite case most of the radiation is reflected back and dissipated within the device itself [Jain et al. 1984; Bindslev Hansen and Lindelof 1984; Konopka 1994].
Another concern are various junction parasitics; for example, junction capacitances are a source of power reduction at higher frequencies [Lukens 1990; Wiesenfeld et al. 1994]. This favors small-area Josephson junctions. Another argument pointing to the same conclusion is increased noise and linewidth broadening in large-area junctions [Kunkel and Siegel 1994; Konopka 1994]. Technically, for w≧4&lgr;
j
, where w is the junction width and &lgr;
j
is the Josephson penetration depth, the current flow becomes inhomogeneous [Kunkel and Siegel 1994]. It has been understood also that to achieve complete phase locking in an array of coupled Josephson junctions, it is necessary that all the junctions within the array have similar critical currents (I
c
); in general, uniformity of ±5% or better is required for linear arrays [see e.g. Konopka 1994].
It is possible to relax somewhat the above stringent requirements by using a distributed arrays of equidistant Josephson junctions (see FIG.
8
), provided that the operating frequency is adjusted in such a way to match the spacing between the junctions with the wavelength of the emitted electromagnetic radiation [Lukens 1990; Han et al. 1994]. This obviously reduces tunability in frequency, while the power of the emitted radiation can be increased significantly.
There have been numerous experimental studies of Josephson junction arrays, and some remarkable results have been achieved. Most of these were based on conventional (low-T
c
) superconductors, e.g. using Nb/Al—AlO
x
/Nb, trilayer junctions. Complete phase locking has been demonstrated in a linear array of 100 such junctions [Han et al. 1993]. In some cases, a broad-band antenna (for example, a bow-tie antenna, or a two-arm logarithmic spiral antenna), was integrated on the chip, and off-chip radiation was detected and measured. In other cases, another Josephson junction was integrated on-chip and coupled via a transmission line to the array. Some of the best results include the following ones. Emission of P=50 &mgr;W at &ngr;=400-500 GHz was observed from a distributed array of 500 Josephson junctions [Han et al. 1994]. In another circuit design (10×10 array), radiation was generated with a linewidth as small as &Dgr;&ngr;=10 kHz, tunable over a broad range, &ngr;=53-230 GHz [Booi and Benz 1994].
With the discovery of high-temperature superconductivity in La—Ba—Cu—O by G. Bednorz and K. A. Müller in 1986, and subsequent improvements of the critical temperature in related cuprate compounds up to T
c
>160 K, great expectations have arosen for superconductive electronics, operational at liquid nitrogen temperature and even above it. Indeed, Josephson junctions have been fabricated since 1987 in dozens of laboratories worldwide, by a variety of techniques. Emission due to ac Josephson currents are artificial high T
c
Josephson junctions was measured and analyzed [Kunkel and Siegel 1994]. In the same study, phase locking of two step-edge junctions was demonstrated over a broad frequency range of &ngr;=80-500 GHz. In larger arrays, only partial (up to 4 junctions) and rather unstable phase locking was observed [Konopka 1994]. This was understood to originate from a generically large non-uniformity of such step-edge high-T
c
Josephson junctions, where critical current variations of ±50% are typical [Konopka 1990]. In another experiment five and ten-junction arrays, one next to the other, were fabricated using step-edge HTS junctions [Kunkel and Siegel 1994], again with only partial phase-locking and very small output power.
Artificial high-T
c
Josephson junctions and stacks are prerequisite in one embodiment of the present invention (see section V). They have indeed been fabricated successfully already [Bozovic et al. 1994, Bozovic and Eckstein 1995, 1996a,b; Eckstein et al. 1992, 1995, Ono et al. 1995] using atomic-layer-bylayer Molecular beam epitaxy (ALL-MBE). A variety of barrier layers have been explored, including Bi
2
Sr
2
CuO
6
[Bozovic and Eckstein 1996b], Bi
2
Sr
2
Dy
x
Ca
1−x
Cu
2
O
8
[Bozovic and Eckstein 1996, 1995], Bi
2
Sr
2
Dy
x
Ca
1−x
Cu
8
O
20
and BiSr
2
Dy
x
Ca
1−x
Cu
8
O
19
[Bozovic and Eckstein 1996, Eckstein et al. 1995], etc. High-resolution cross-sectional electron microscopy has shown virtually atomically perfect interfaces between the barriers and the superconducting electrodes [Bozovic et al. 1994b]. These multilayers were lithographically processed into mesa structures for vertical transport devices [Eckstein et al. 1992, Bozovic and Eckstein 1996b]. Both proximity-effect (SNS) junctions [Bozovic and Eckstein 1996b, 1995] and tunnel (SIS) junctions [Bozovic and Eckstein 1996a, Bozovic et al. 1994] have been fabricated in this way. They have shown remarkably high characteristic voltages, up to I
c
R
N
=10 mV (which corresponds to &ngr;≈2.5 THz) and uniformity of better than ±5% [Bozovic and Eckstein 1996a]. It was further demonstrated that the barrier properties such as its normal state resistance R
N
and critical current I
c
can be engineered over a very broad range (four orders of magnitude) by varying the doping level within the barrier, e.g., by varying x in the barrier layer consisting of Bi
2
Sr
2
Dy
x
Ca
1−x
Cu
8
O
20
[Bozovic and Eckstein 1996a,b, 1995, 1994a; Eckstein et al. 1992]. Finally, some short vertical stacks of such Josephson junctions have already been fabricated and they showed perfect phase locking [Bozovic and Eckstein 1996b, 1994a; Eckstein et al. 1995; Ono et al. 1995]. In conclusion, every critical technological step related to fabrication of artificial trilayer Josephson junctions, and their vertical stacks, which we assumed to be feasible in Section V (iv). below, has already been successfully demonstrated and reduced to p
Merchant & Gould P.C.
Oxxel Oxide Electronics Technology GmbH
Wilson Allan R.
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