Phonon assisted tunneling through single crystals of GaS and GaSe

of 5 /5
Physica 105B (1981) 329-333 North-Holland Publishing Company PHONON ASSISTED TUNNELING THROUGH SINGLE CRYSTALS OF GaS AND GaSe K. YAMAGUCHI and Y. NISHINA The Research Institute [or Iron, Steel and Other Metals, Tohoku University, Sendai 980, Japan Tunneling spectra have been obtained for Pb/GaS/Pb and Pb/GaSe/Pb junctions at 1.8 K for the normal state of Pb in the external magnetic field as well as for the superconducting state. The insulating barrier of GaS or GaSe, whose thickness is several unit layers, is cleaved from a single crystal. The observed structures in d2V/dl 2 characteristics have been interpreted in terms of inelastic electron tunneling due to the coupling between electrons and phonons of the barrier. The main tunneling structures show quantitative agreement with the energies of phonons having the wave vectors near the edges of the Brillouin zone rather than its center with our sensitivity of measurement. The relative line intensity and its bias voltages of the d2V/dl 2 structures do not agree quantitatively with the density of states of phonons calculated previously. 1. Introduction Inelastic electron tunneling (lET) through a metal-insulator-metal (MIM) junction was reported in 1966 by Jaklevic and Lambe [1]. The observed spectrum was analyzed in terms of the interaction of the tunneling electrons with the vibrations of organic molecules adsorbed on the metal oxide surface. Since then, several groups [2] have measured IET spectra to investigate the mechanism of the electron-phonon interaction in the insulating layer. The insulator in the MIM junction consists either of metal oxides or of evaporated insulating films. The quantitative analysis, however, encounters a serious difficulty in comparing IET spectra with the phonon ener- gies deduced from other experimental methods and with the density of states of phonons be- cause there is an ambiguity in the chemical com- position of the insulating layer as well as in the polycrystalline/amorphous structures of the films. The spectral analysis of IET may give more reli- able information if the insulating layer consists of a single crystal. Rau luszkiewicz and Nishina [3, 4] have shown that GaSe and PbI2 have great ad- vantage for this purpose because the layered structure of these materials makes the crystal easily cleavable. They attributed the observed structures to lET due to the interaction with lattice vibrational modes and to the resonant tunneling through localized electronic states. The present paper reports further experimen- tal investigation of the inelastic tunneling spectra in GaS and GaSe. Particular attention has been paid to symmetries of phonons participating in the scattering of tunneling electrons. Since the above two materials have an isomorphic layer structure, apart from polytypism, we may derive characteristics general, or at least common, to III-VI layer compounds in the IET phenomena. The structures of the tunneling spectra have been compared with the phonon energies deduced from several optical measurements [5- 13] and inelastic neutron scattering data [14-16] as well as with the calculated density of states of phonons [14, 15]. The comparison shows that phonons lying near the zone boundary pre- dominate IET. 2. Experimental procedures Single crystals of ~-GaS and e-GaSe were grown by the Bndgman method. The fabrication technique of MINI structures was similar to the methods employed by Kurtin et al. [17] and RauJ~uszkiewicz and Nishina [3, 4]. Single crystal pieces of GaS and GaSe about 0.5 mm thick and 5 x 10 mm 2 were cleaved from a large ingot. The cleaved surface was covered by a vacuum- evaporated (at about I x 10-6 Torr) film of more than 2000/I, thick Pb, AI or Cu. The sample surface with the evaporated film was fixed on a 0378-4363/81/0000-0000/$2.50 ~) North-Holland Publishing Company and Yamada Science Foundation

Transcript of Phonon assisted tunneling through single crystals of GaS and GaSe

Page 1: Phonon assisted tunneling through single crystals of GaS and GaSe

Physica 105B (1981) 329-333 North-Holland Publishing Company

PHONON ASSISTED TUNNELING THROUGH SINGLE CRYSTALS OF GaS AND GaSe

K. YAMAGUCHI and Y. NISHINA The Research Institute [or Iron, Steel and Other Metals, Tohoku University, Sendai 980, Japan

Tunneling spectra have been obtained for Pb/GaS/Pb and Pb/GaSe/Pb junctions at 1.8 K for the normal state of Pb in the external magnetic field as well as for the superconducting state. The insulating barrier of GaS or GaSe, whose thickness is several unit layers, is cleaved from a single crystal. The observed structures in d2V/dl 2 characteristics have been interpreted in terms of inelastic electron tunneling due to the coupling between electrons and phonons of the barrier. The main tunneling structures show quantitative agreement with the energies of phonons having the wave vectors near the edges of the Brillouin zone rather than its center with our sensitivity of measurement. The relative line intensity and its bias voltages of the d2V/dl 2 structures do not agree quantitatively with the density of states of phonons calculated previously.

1. Introduction

Inelastic electron tunneling (lET) through a metal-insulator-metal (MIM) junction was reported in 1966 by Jaklevic and Lambe [1]. The observed spectrum was analyzed in terms of the interaction of the tunneling electrons with the vibrations of organic molecules adsorbed on the metal oxide surface. Since then, several groups [2] have measured IET spectra to investigate the mechanism of the electron-phonon interaction in the insulating layer. The insulator in the MIM junction consists either of metal oxides or of evaporated insulating films. The quantitative analysis, however, encounters a serious difficulty in comparing IET spectra with the phonon ener- gies deduced from other experimental methods and with the density of states of phonons be- cause there is an ambiguity in the chemical com- position of the insulating layer as well as in the polycrystalline/amorphous structures of the films. The spectral analysis of IET may give more reli- able information if the insulating layer consists of a single crystal. Rau luszkiewicz and Nishina [3, 4] have shown that GaSe and PbI2 have great ad- vantage for this purpose because the layered structure of these materials makes the crystal easily cleavable. They attributed the observed structures to lET due to the interaction with lattice vibrational modes and to the resonant tunneling through localized electronic states.

The present paper reports further experimen- tal investigation of the inelastic tunneling spectra in GaS and GaSe. Particular attention has been paid to symmetries of phonons participating in the scattering of tunneling electrons. Since the above two materials have an isomorphic layer structure, apart from polytypism, we may derive characteristics general, or at least common, to III-VI layer compounds in the IET phenomena.

The structures of the tunneling spectra have been compared with the phonon energies deduced from several optical measurements [5- 13] and inelastic neutron scattering data [14-16] as well as with the calculated density of states of phonons [14, 15]. The comparison shows that phonons lying near the zone boundary pre- dominate IET.

2. Experimental procedures

Single crystals of ~-GaS and e-GaSe were grown by the Bndgman method. The fabrication technique of MINI structures was similar to the methods employed by Kurtin et al. [17] and RauJ~uszkiewicz and Nishina [3, 4]. Single crystal pieces of GaS and GaSe about 0.5 mm thick and 5 x 10 mm 2 were cleaved from a large ingot. The cleaved surface was covered by a vacuum- evaporated (at about I x 10 -6 Torr) film of more than 2000/I, thick Pb, AI or Cu. The sample surface with the evaporated film was fixed on a

0378-4363/81/0000-0000/$2.50 ~) North-Holland Publishing Company and Yamada Science Foundation

Page 2: Phonon assisted tunneling through single crystals of GaS and GaSe

330 K. Yamaguchi and Y. Nishina/Phonon assisted tunneling through GaS, GaSe

cover glass plate with thinner-free silver paste. The opposite side of the sample was peeled away by repetitively attaching the Scotch tape and removing the pasted part of the sample. The junction fabrication was completed by evaporat- ing Pb again on the thin film of the sample. The diameter of the evaporated film was ap- proximately 0.18mm. This arrangement of the junction can stand the temperature cycling be- tween 1.8 K and room temperature at least five times.

The choice of appropriate junctions for tunne- ling measurements was determined in ac- cordance with I - V characteristics in the super- conducting state of Pb as well as in the normal state. Some of our junctions with their resis- tances smaller than about 100/2 showed several unreproducible peaks in d V/dl below the critical temperature of Pb and in the magnetic field lower than the critical field even if the tunneling current was more dominant than the leakage current. According to the temperature depen- dence and the magnetic field dependence of these peaks in d V/dI, they may be related to the parallel or series resistances caused by thin Pb channels through pinholes or by thin electrodes of Pb. Such junctions were excluded from our measurements. The appropriate junctions were obtained throughout about 20 junctions among 1000 fabricated by the method mentioned above. The resistances of such junctions were in the range from 1 to 100 O at 1.8 K. The junctions had a thickness from 10 to 30 A, as estimated from the I - V characteristics [18]. In this paper we report the results of Pb/GaS (or GaSe)/Pb junctions. In the junctions fabricated by our method we have not been able to reproduce the previous results by Rau/uszkiewicz and Nishina [3,4].

The characteristics of d V / d I - V and d2V/dI 2 - V were measured at 1.8 K by means of the conventional constant current modulation circuit. The modulation voltage across the junction was set to 1 mV peak-to-peak at V = 0. A magnetic field of about 3 kG was applied in the direction of tunneling current to destroy the superconduc- ting state of Pb when we observed the normal tunneling characteristic.

3 . R e s u l t s

Fig. l(a) shows the I - V curve of the Pb/GaS/Pb junction at 1.8 K and in the external field of about 30 G, which exhibits the well-

1.5

< 1.0 E

0..5

I - - z 0 IzJ o,-

t,.. -0.5 D ¢ J

- 1.0

- I . 5

I I

(a)

- I 0 - 8 - 6 - 4 - 2

6.8

6.7

6.6

6.5

-o 6.4

6.3

I I I I

" 1 I I I I

I I I I

I

I i i

A V II\I T IJ vV

- 5 0 - 4 0 - 30 - 20 - I 0 0 I0 20 30 40 50 BIAS VOLTAGE ( m V )

Fig. 1. (a) I vs. V of a Pb/OaS/Pb junction at 1.8 K and at about 30 G in the superconducting state of lab. Similar results are also attained for a Pb/GaSe/Pb junction. (b) dV/dI vs. V of the same junction as (a) at 1.8 K and at 3.2 kG in the normal state of Pb. (c) d2V/d/2 vs. V of the same junction as (a) at 1.8K and at 3.2kG. The inset in the figure is d2V/d/2 vs. V - 2Ale of the same junction as (a) at 1.8 K and at about 30 G. 2A = 2.6 mV is the superconducting energy gap of Pb at 1.8K.

Page 3: Phonon assisted tunneling through single crystals of GaS and GaSe

K. Yamaguchi and Y. Nishina/Phonon assisted tunneling through GaS, Gaffe 331

known superconducting characteristic of Pb. Figs. l(b) and 1(c) show the dV/dI and d2V/dI 2 curves of the same junction at 1.8 K and in the field of 3.2 kG, respectively. The steps in dV/dI or the peaks in d 2 V ] d l 2 are observed at the bias voltages as shown in tables I and II for GaS and GaSe, respectively. The determination of the bias voltage is confirmed by the dEV/dl 2 charac- teristic at 30 G with the resolution better than that at 3.2kG because the presence of the superconducting energy gap reduces the thermal broadening in the electron energy distribution. The line shape of the spectrum, however, is modified by the density of states of Pb.

4. Discussion

The interpretation of the structures observed in d2V]d/2 of the junction with the barrier of GaS or GaSe is given in terms of the interaction of tunneling electrons with lattice vibrational modes of the insulating layer and the electrode.

Table I Comparison of the tunneling and optical spectra in GaS

Phonon energy (meV)

Present work IR Raman MCP IET (1.8 K) (3O0 K) [10, 11] (3O0 K) [9] (4.2 K) [5--7]

2 .7E~ 9.2 Elg

10.9 ± 0.5 (w) 10.7 ± 0.2 13.4 (s) 21.1 (s) 21.8 23.4 (w) 23.3 A[g 27.4 (w) 31.3 (v.w.)

36.6 (w) 36.6 E~u (TOy Lb 36.1 E2g

36.7 E~ 38.3 (m) 38.1

39.5 Aiu (TO) b 41.8 A ~ (LO) b

42.8 (s)

45.7 (m) 44.6 E~u (LOy 'b

45.0 A~g

42.6

46.7

"Ref. 10. b Ref. 11. s = strong, m = medium, w = weak, v.w. = very weak.

Table II Comparison of the tunneling and optical spectra of GaSe

Phonon energy (meV)

Present work IR Raman MCP lET (1.8 K) (300 K) [12, 13] (300 K) [9] (4.2 K) [8]

2.2E a 7.3E 'a

9 .3±0.5(s) 14.1±0.1

15.6 ~) 15.~ 16.7 A~ l 17.5 A~ 2

20.2 (m) 20.3" 21.7

26.0 (w) 26.0

26.2 E '3 (TO) b

27.5 (m)

3O.0 (w) 29.4 A~ 3 (TO) c

30.5 A~ 3 CLOy

26.1 E "4

26.6 E '4

30.6 A~ 3 (LO)

31.1 (m) 31.5 E '3 (LO) b 31.5 E '3 (LO)

33.7 (v.w.) 37.8 (w) 38.1 Ai '

31.0

aT he phonon energies extrapolated to x = 0 from the energies of MCP in y-GaSxSel-x, with 0.01 ~ x ~ 0.4 [8].

b Ref. 12. c Ref. 13.

In fig. 2(c) the peaks at V = 5.6 and 9.3 mV for the Pb/GaS/Pb junction are due to Pb phonons [19]. In the spectrum of fig. 3(c) for the Pb/GaSe/Pb junction, the peak at 5.7 mV cor- responds to the Pb phonons and the peak at 9 .3mV consists of a superposition of the phonons of GaSe (9.2 mV at point M [15]) and Pb.

Tables I and II show the energies of the peaks o f d E V ] d l 2 c u r v e s o f G a S a n d GaSe, respectively, in comparison with the phonon energies deduced from measurements of Raman scattering [9], IR reflection/absorption [10-13], as well as optical absorption/photoluminescence near the band edge [5--8]. The d2V/d/2 peaks at 15.7 and 20.2 mV disagree with the energies of momen- tum conserving phonons (MCP), i.e. 14.1 and

Page 4: Phonon assisted tunneling through single crystals of GaS and GaSe

332 K. Yamaguchi and Y. Nishina/Phonon assisted tunneling through GaS, GaSe

0 . 5 n-

O

t J hl >

W

a ILl

O 5

~ 3

I

0

-mDo• •

, - ' e l • o

, , qoo • •

• • • I J •

: : 4 ". I o ":%° o ' .

i ~ o • a log o • o °

o •

[] • o

~.~._- ': I

I

(o)

A

(/3 I-" H

z 3

n,- <C

- 2 l I

0 I0

(c)

I

20 ENERGY

3 0 4 0 5 0 ( m e V ]

Fig. 2. (a) The experimental data of the inelastic neutron scattering for GaS by Polian et al. [16]. (b) Phonon frequency distribution function calculated by Powell et al. [14]. (c) d2V/d/2 vs. V of the Pb/GaS/Pb junction at 1.8K and at 3.2 kG.

- u p • • •

o (0) - i n • •

- 1 • • • o o •

a,m - • • • • •

~ o ~ - + - -

m I l I ~ O OO I I O

( ~ - i n • • go

L • e •

0 . 5 ~ - - ± - -

N °

A

U) I - -

~5 n~

>

% i

I

0 IO

( c )

J I '3" 2 0 3 0 4 0

E N E R G Y ( m e V )

[..,

Fig. 3. (a) The experimental data of the inelastic neutron scattering for GaSe by Jandl et al. [15]. (b) Phonon frequency distribution function calculated by Jandl et al. [15]. (c) d2V/dI 2 vs. V of the Pb/GaSe/Pb junction at 1.8 K and at 3.2 kG.

21.7 meV [8], deduced from photoluminescence due to indirect bound excitons for e-GaSe. In y-GaSxSel-x crystals for 0.01 ~ x ~ 0.4 grown by the Bridgman method [20], the energies of MCP extrapolate to 15.4 and 20.3 meV for the hypo- thetical y-GaSe [8]. The present results in the Ph/GaSe/Pb junction agree well with the latter values. Results of inelastic neutron scattering [14-16] are also compared with our data in figs. 2 and 3. The main peaks of the tunneling spectra show better agreement with the phonon energies

near the edges (i.e. the faces including point M) of the Brillouin zone rather than those near the zone center. Phonons at F point correspond only to minor peaks. Hence, the spectra cannot be explained in terms of the phonon dispersion relationship along the kz direction (i.e. the direction of a wave vector perpendicular to a basal plane or a tunneling barrier). One may note that the relative line intensities in d 2 V / d I 2 and the bias voltages at the peaks do not agree quantitatively with the density of states curves

Page 5: Phonon assisted tunneling through single crystals of GaS and GaSe

K. Yamaguchi and Y. Nishina/Phonon assisted tunneling through GaS, GaSe 333

calculated by Powell et al. for GaS [14] and by Jandl et al. [15], respectively. For the dis- crepancy one should take the following into ac- count. First, a simple interatomic force model, including short-range forces only, is fitted to the phonon energies to calculate the density of states. Polian et al. [16] suggest that no simple model can be expected to give a complete ac- count of all phonon branches simultaneously, since GaS is covalent and ionic at the same time. This comment may be also applicable to GaSe. Secondly, the intensity of the tunneling spectrum depends upon the electron-phonon interaction as well as the density of states of phonons, as suggested by Bennett et al. [21]. Thirdly, since the insulating layer has a thickness in the range of several unit layers, the d2V/dI 2 curves would reflect a size effect on the density of states of phonons.

No spectral structure appears in the bias range from 50 to 500 mV. This fact shows the absence of detectable amounts of organic molecules ad- sorbed on the insulating plane.

Acknowledgements

We would like to thank Dr. J. Rauiuszkiewicz for his encouragement of this study and Dr. Y. Sasaki for a valuable discussion of the results.

Rderences

[1] R.C. Jaklevic and J. Lambe, Phys. Rev. Lett. 17 (1966) 1139.

[2] E.L. Wolf, in: Solid State Physics, vol. 30, H. Ehren- reich, F. Seitz and D. Turnbull, eds. (Academic Press,

New York, 1975) p. 1. The reports preceding this article are summarized in the references herein.

[3] J. Ratd'uszkiewicz and Y. Nishina, in: Physics of Semi- conductors 1978, B.L.H. Wilson, ed. (Inst. Phys. Conf. Ser. No. 43, 1979) p. 757.

[4] J. Rauluszkiewicz and Y. Nishina, Physiea 99B (1980) 371.

[5] E. Aulich, J.L. Brebner and E. Mooser, Phys. Stat. Sol. 31 (1969) 129.

[6] B.S. Razbirin, V.P. Mushinskil, M.I. Karaman, A.N. Starukhin and E.M. Gamarts, Sov. Phys. Semiconduc- tors 12 (1978) 19.

[7] K. Yamaguchi and Y. Nishina, unpublished. [8] Y. Sasaki, H. Serizawa and Y. Nishina, to be published. [9] M. Hayek, O. Brafman and R.M.A. Lieth, Phys. Rev.

B8 (1973) 2772. [10] N. Kuroda and Y. Nishina, Phys. Rev. B19 (1979) 1312. [11] V. Riede, H. Neumann, H.X. Nguyen, H. Sobotta and

F. Levy, Physica 100B (1980) 355. [12] N. Kuroda, Y. Nishina and T. Fukuroi, J. Phys. Soc.

Japan 28 (1970) 981. [13] P.C. Leung, G. Andermann, W.G. Spitzer and C.A.

Mead, J. Phys. Chem. Solids 27 (1966) 849. [14] B.M. Powell, S. Jandl, J.L. Brebner and F. Levy, J.

Phys. C: Solid State Phys. 10 (1977) 3039. [15] S. Jandl, J.L. Brebner and B.M. Powell, Phys. Rev. B13

(1976) 686. [16] A. Polian, M. Kunc, R. Le Toullec and B. Dorner, in:

Physics of Semiconductors 1978, B.L.H. Wilson, ed. (Inst. Phys. Conf. Ser. No. 43, 1979) p. 907.

[17] S.L. Kurtin, T.C. McGill and C.A. Mead, Phys. Rev. B3 (1971) 3368.

[18] J.M. Rowell, in: Tunneling Phenomena in Solids, E. Burstein and S. Lundgvist, eds. (Plenum Press, New York, 1969), p. 385.

[19] J.M. Rowell, W.L. McMillan and W.L. Feldmann, Phys. Rev. 180 (1969) 658.

[20] H. Serizawa, Y. Sasaki and Y. Nishina, J. Phys. SOc. Japan 48 (1980) 490.

[21] A.J. Bennett, C.B. Duke and S.D. Silverstein, Phys. Rev. 176 (1968) 969.