Volume 25A, number 7 PHYSICS L E T T E R S 9October 1967

A N I M P U R I T Y - C O N C E N T R A T I O N D E P E N D E N T Z E R O B I A S T U N N E L L I N G A N O M A L Y

F . M E Z E I Institute for Experimental Physics of Roland E~tvSs University, and Central Research Institute for Physics. Budapest, Hungary

Received 5 September 1967

The addition of different amounts of chromium impurities to the insulating layer of AI-I-A1 junctions have been found to cause different types of zero bias anomalies. The dynamical res istance versus bias curves have shown a large maximum and a slight minimum as well, depending on impurity concentration.

Recent ly ze ro bias tunnell ing anomal ies have been obse rved on s e v e r a l junctions of va r ious m a t e r i a l s [1-3]. The anomal ies could be under - stood as effects of pa ramagne t i c impur i t i e s at the insulat ing l aye r [4-6], which idea is supported by the p resen t work. We have inves t iga ted A1-A1203-AI junctions with Cr impur i t i e s in t ro - duced into the b a r r i e r region.

The junctions were p r e p a r e d by evaporat ion in a p r e s s u r e of 2 × 10 -5 H g m m , using high puri ty A1 (99.999%) and Cr (Johnson and Matthey, 99.9990/0). The f i r s t deposi ted A1 f i lm was oxi- dised in a tmosphe r i c a i r for 5-10 min. We put Cr ions on the sur face of this oxide l a y e r by eva - porat ion at a ra te of the o rde r of 0.1 A / s e c , the amount of impur i t i e s in t roduced was contro l led by the exposi t ion t ime. The Cr was probably in par t oxidised dur ing deposit ion. A second A1 f i lm was evapora ted immedia te ly t he rea f t e r to c o m - plete the junction. The junction a r e a was 0.1 ram2 and the room t e m p e r a t u r e junction r e s i s t a n c e of the o rde r of 10 ohm.

The dynamical r e s i s t i v i t y v e r s u s vol tage was m e a s u r e d at 1.6°K using the s tandard constant- cu r r en t method and modulat ion techniques .

Fig. 1 shows some typical plots R(V)/R(O) v e r s u s vol tage. The following c h a r a c t e r i s t i c fea - tu re s a r e shown up:

1) At high impur i ty concent ra t ions ( seve ra l a tomic l ayers ) we have the well-known s y m m e - t r i c r e s i s t i v i t y max imum found by Rowell et al. for C r - C r O - A g junctions [2] and explained by an t i f e r romagne t i c coupling between tunnell ing e l ec t rons and impur i ty a toms [7,8]. The width of our r e s i s t i v i t y peak i n c r e a s e s with dec reas ing impuri ty concentra t ion.

2) At low impur i ty concentra t ions (about one

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c=

-3

z

=< 0.5

z

0 I , , I -~oo -4oo ~ ~-4oo ,-2oo

VOLTAGE (,,,,V)

Fig. 1. Normalized dynamical resistance R(V)/R(O) versus voltage at 1.6°K for A1-A1203-A1 junctions con- taining different amount of Cr impurities in the barr ier region: a) no impurity, b), c) and d) Cr impurity intro- duced by evaporation for 10, 25 a~d 120 sec , respec- tively, at a rate of the order 0 .1A/sec. The curves a), b) and c) are shifted upwards by 0.3, 0.2 and 0.1 unity,

respectively.

Volume 25A, number 7 P H Y S I C S L E T T E R S 9 October 1967

a tomic layer ) we find the c h a r a c t e r i s t i c sl ight r e s i s t i v i t y min imum at z e r o bias superposed on a wide, a s y m m e t r i c m a x i m u m . The width of the min imum i n c r e a s e s with d e c r e a s i n g impur i ty concentra t ion. This type of anomaly have been found e a r l i e r on Ta-TaO-Ag(A1) junctions [2] and an explanat ion by an t i f e r romagne t i c [4,5] or f e r - r omagne t i c [6,7] coupling has been proposed.

3) The background cu rve is a s y m m e t r i c . This a s y m m e t r y alone may be respons ib le for the a s y m m e t r y of the plots of the second type.

To explain the above r e s u l t s s e v e r a l poss ib i l i - t ies may be cons idered . The addit ional effect of o ther impur i t i e s than Cr was ru led out except those contained in our high puri ty Cr m a t e r i a l , so these may be r e g a r d e d as being unimportant . In the case of low concent ra t ions any o rde r ing of the impur i ty a toms is r a t h e r improbable . On the o ther hand c u r v e s of type 1 have been found by Wyatt et al . for Cu and Ti impur i t i e s , for which no magne t ic o rde r ing is excepted [3]. Thus o r - der ing may not be impor tan t in causing sharp r e - s i s t iv i ty max ima . In our opinion the re a re two poss ib i l i t i e s and both give a good qual i ta t ive unders tanding of expe r imen t a l r e su l t s on the base of S61yom and Zawadowski ' s theory [7], des - c r ib ing the effect as a change in the density of s ta tes of e l ec t rons at the b a r r i e r , cons ider ing the Kondo sca t t e r i ng in a s e l f - cons i s t en t way:

a) T h e r e is an an t i f e r rom agne t i c coupling and the sca t t e r ing ampli tude has a max imum at s e v e r a l meV above the F e r m i energy. Recent theo re t i ca l ca lcula t ions , however , p red ic t such max imum much c l o s e r to the F e r m i leve l (at about an e n e r - gy kT) [9]. b) The coupling can he a n t i f e r r o m a g - net ic or f e r r o m a g n e t i c accord ing to the posi t ion of the Cr ions in the b a r r i e r region. F e r r o m a g n e t ic coupling might be expected for Cr ions inside the oxide layer .

It is p l easu re to thank Prof . E. Nagy and Prof . L. Pal for r ender ing poss ib le this work and for the i r continuous in te res t . I should like a lso to thank Dr. A. Zawadowski and J. S61yom for many s t imula t ing d i scuss ions .

1. A.F.G.Wyatt, Phys. Rev. Letters 13 (1964)401. 2. J. M. Rowell and L. Y. L. Shen, Phys. Rev. Letters

17 (1966) 15. 3. A. F. G. Wyatt and D. J. Lythall, to be published. 4. P.W. Anderson, Phys. Rev. Letters 17 (1966) 95. 5. J. Appelbaum, Phys. Rev. Letters 17 (1966) 91;

Phys. Rev. 154 (1967)633. 6. A. Zawadowski, Proc. Xth Int. Conf. on Low Temp.

Physics, Moscow, 1966, to be published. 7. J. S61yom and A. Zawadowski, KFKI report 14/1966,

and to be published. 8. J. Appelbaum, J. C. Phillips and G. Tzouras, to be

published. 9. H. Suhl and D.Wong, Physics 3 (1967) 17.

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S A T U R A B L E T R A N S M I S S I O N BY M U L T I P H O T O N A B S O R P T I O N I N S E M I C O N D U C T O R S *

R. BRAUNSTEIN Department of Physics. University of California at Los Angeles,

California. USA

Received 5 September 1967

Multiphoton absorption can set an intrinsic upper limit to the power density that can be transmitted through semiconductors. This mechanism is an effective means of optically pumping semiconductor laser and can limit the power density from such devices, or can enable the fashioning of nonlinear optical power limiters.

Recent m e a s u r e m e n t s of double-photon abso rp - tion in a number of s emiconduc to r s such as CdS

* This research is part of Project Defender Contract NONR 233(93) from the Office of Naval Research, the Advanced Project Agency and the Department of De- fense of the United States of America.

p,2] , GaAs [3], VbTe [4,5], InSb [4] and ZnS [6] have shown that the theory [1,7] developed for the two-photon exci ta t ion of in terband t rans i t ions is adequate to account for the obse rved c r o s s sec t ions .

The p rev ious theory [1,7], developed for the double-photon absorpt ion in semiconduc to r s u t i l -

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