Observation of phonons in Bismuth by inelastic electron tunneling

Observation of phonons in Bismuth by inelastic electron tunneling

Solid State Communications,Vol. 15, pp. 1639—1642, 1974. Pergamon Press. Printed in Great Britain OBSERVATION OF PHONONS IN BISMUTH BY INELASTIC EL...

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Solid State Communications,Vol. 15, pp. 1639—1642, 1974.

Pergamon Press.

Printed in Great Britain

OBSERVATION OF PHONONS IN BISMUTH BY INELASTIC ELECTRON TUNNELING* J. Straust and J.G. Adler Department of Physics, University of Alberta, Edmonton, Alberta, Canada (Received 22 July 1974 by R. Barrie)

We have observed phonons in polycrystalline Bismuth films by inelastic electron tunneling measurements with Bi/Bi, Mg/Bi and Al/Bi tunnel junctions.

IN THIS LETTER we wish to report the observation of phonons in polycrystalline Bi films by electron tunneling measurements with Bi/Bi, Mg/Bi and 41/Bi junctions. Some of these phonons were first observed by Esaki et al.1 in single crystal junctions. A few of these phonons have also been observed by Rowell eta!.2 in evaporated Bi films. Our studies augment the previously published data as well as providing better resolution of some of the phonon peaks.

material. To enhance large grain formation hot substrates (80°C) were used. Even when Bi films are evaporated on glass slides the trigonal axis (for most grains) is usually within 20°of the film normal.4 For Bi condensed on mica, the appearance of mosaic single crystals, with the trigonal axis perpendicular to the substrate has been observed.5’6 Electron microscopy showed that the lateral size of microcrystals in our films is of the order of 1500—2000 A independent of the evaporation rate.

The appearance of a structure in tunneling characteristics (a = dud V, da/d V = d21/d V2) of normal metal—insulator—metal junctions due to inelastic excitation of phonons in the metal electrodes or the barrier layer is well established.2’3 Since a phonon excitation provides an additional channel for tunneling electrons the effect manifests itself as a small (less than 1 per cent) increase in the conductance of the junction at eV = hw (where w is the characteristic phonon frequency). Measurement of da/dV provides the most convenient way to observe these small conductance changes.

The insulating layers of Al/Bi and Mg/Bijunctions were prepared by glow discharge techniques.7 In the case of a Bi base layer this method of oxidation proved unsatisfactory, and the oxide layer had to be thermally grown at 200°Cin air for two hours. Any annealing effect that the thermal oxidation may have had did not result in any detectable change in crystal structure. Junctions with Pb cover layers were made to verify tunneling by the observation of an energy gap. Such cover layers are not suitable for Bi phonon studies since the Pb phonons have similar low energies.

Evaporation of Bi films was carried out in a vacuum system at pressures l07torr. Clean glass slides or freshly cleaved mica served as substrate

*

The measurements were carried out by a combination of bridge techniques and harmonic detection.8 The system was interfaced to a minicomputer for fast evaluation of incoming data. The details of the measuring system will be published elsewhere.

This wokwas supi rtedmpatbygrants from the

~ Present address: J. Straus, Bell Canada Northern Electric Research Limited, Ottawa, Canada. 1639

1640

OBSERVATION OF PHONONS IN BISMUTH

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Vol. 15, No. 10

4, -~

532 I Al/B~ 08 K

16

ENERGY (n,eV)

FIG. I. dUe/dVvS energy for a Bi/Bi junction (prepared on mica). The numerals show the location of phonon peaks given in Table I.

I 3

I 7

9

II

ENERGY

3

15

i~

IS

meV)

FIG. 3. dae/dVvs energy for an Al/Bi junction (prepared on a glass slide). The numerals show the location of phonon peaks given in Table 3.

4

038 Mg/B 108K .=I

El

g,

E

peak possible peaks in Mg/Bi cryst. Table 2. position Identification ofphonon number (meV) identification direction

I>

1

3.3

LA

2 fold axis

2 3 4 65

7.7 9.7 12.4

LA TO TO, LO

3 fold axis 3 fold axis 3 fold axis

0

_______

0 0

5

0

FNERGY IS 20 (~eVl 25

30

35

40

FIG. 2. Observation of inelastic phonon assisted tunneling in Mg/Bi. (prepared on glass slide). The

numerals show the location of phonon peaks given in Table 2. The unmarked low energy peak is due to bias anomaly. Table 1. Identification of phonon peaks in Bi/Bi junction ________________________________________________ peak number

2 3 4

position (meV) 3.1 4.8 7.8 12.3

possible identification LA TA LA TO, LO

cryst. direction 2 fold 3 fold 3 fold 3 fold

axis axis axis axis

Since the inelastic contribution to a is an even function of the junction bias it is more convenient to

27.5 18.4

3’” Transverse MgMg3”1 Longitudinal

Table 3. Identification ofphonon peaks in Al/Bi. An energy of O.l8meV(the halfgap ofAl) has been subtracted from the raw peak positions peak number

position (meV)

2

3 4

5

possible identification

cryst. direction

3.7

LA

6.5 8.5 11 .4 15.1

TA LA LO 2 phonon processes

2 fold axis 3 fold axis 3 fold axis 2 fold axis

reduce the data in terms of the even part of the conductance ~e = ~[a(V) + u(— V)]. Typical results for dUe/d V for Bi/Bi junctions are shown in Fig. 1. Possible assignments of phonon

Vol. 15, No. 10

OBSERVATION OF PHONONS IN BISMUTH

emission thresholds are given in Table 1. Similar identification for Mg/Bi is given in Table 2 and Fig. 2, and the Al/Bi results are displayed in Table 3 and Fig.3.

1641

2.?

The peak at 8.5 meV (denoted as 3) in Fig. 3 is above the LA phonon (7.3 meV) emission threshold. At 4.2 K both peaks (2 and 3) merged into a broad shoulder around 8 meV.

.8

b

A common feature of all three types of junctions is the appearance of phonon peaks in the vicinity of 3 and 12 meV. This suggests that the coupling between electrons and phonons is strongest at these energies. This conclusion agrees with observation of Rowell et al.2 on Al/Bi junctions. We expect that the resolution of phonon peaks at different energies depends on the size and orientation of grains comprising the evaporated Bi film. Finally we would like to mention in passing that all Al/Bi and Mg/Bi junctions exhibited the characteristic W-shape at high energies~”3(Fig. 4). The Bi/Bi junctions on the other hand show the more parabolic shape associated with symmetrical tunnel junctions. We feel that the W-shape arises from the large difference in Fermi energies between the base layer and counter-electrode. Studies of this effect along with identification of band edges are currently in progress and will be subject of a forthcoming paper.

A

Al/Ri

~

-~t~.~10

i~ ENERGY

IS

210

310 ~

[,~~eV]

FIG. 4. Normalized conductance as a function of energy at high bias for the three types ofjunctions discussed in text. The Mg/Bi curve has been displaced upwards by 0.2 for clarity.

Acknowledgements We wish to acknowledge our gratitude to Dr. P. Turner for carrying out extensive electron microscope studies of our Bi films. —

REFERENCES 1.

ESAKI L., CHANG L.L., STILES P.J., O’KANE D.F. and WISER N.,Phys. Rev. 167,037(1968).

2. 3. 4.

JACKLEVIC R.C. and LAMBE J.,Phys. Rev. Lett. 17, 1139 (1965); ROWELL J.M., MCMILLAN W.L. and FELDMAN W.L.,Phys. Rev. 180, 658 (1969). ADLER J.G.,Phys. Lett. 29A, 675 (1969). KOMNIK Y.F. and BUKHSTAB E.I.,Sov. Phys. JETP 27,34(1968).

5. 6.

SHEFTAL R.N., PALATNIK Y., LUTSKII V.N. and ELINSON M.I., Soy. Phys. Dokl. 13,462 (1968). DUGGAL V. and RUPP R., J. Appl. Phys. 40,492 (1968).

7.

MILES J.L. and SMITH P.H.,J. Appl. Phys. 34, 2109 (1963).

8. 9.

ADLER J.G., CHEN T.T. and STRAUS J.,Rev. Sci. Instr. 42, 362 (1971). ADLER J.G. and STRAUS J. (to be published).

10. 11.

KOTOV B.A., OKUNEVA N.M. and PLACHENOVA E.L.,Sov. Phys. Solid State 11, 1615 (1970). YOUNG J.A. and KOPPEL J.V.,Phys. Rev. 134, 1476 (1964); KLEIN J., L~GERA., BELIN M., DEFORNAU D. and SANGSTER M.J.L.,Phys. Rev. 7,2336 (1973).

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OBSERVATION OF PHONONS IN BISMUTH

Vol. 15, No. 10

12.

HAUSER J.J. and TESTARDI L.R.,Phys. Rev. Lett. 20,12 (1968).

13.

VAISNYS J.R., MCWHAN D.B. and ROWELL J.M.,J. Appi. Phys. 40, 2623 (1969).

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