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Au interface at liquid nitrogen temperature
Volume 80A, number 1
PHYSICS LETTERS
10 November 1980
EVIDENCE OF INTERMIXING AT Si(llI)/Au INTERFACE AT LIQUID NITROGEN TEMPERATURE I. ABBATI and L. BRAICOVICH Istituto di Fisica del Politecnico di jltfilano, Milan, Italy
and A. FRANCIOSI 1stituto di Fisica deli’ Università di Roma, Rome, Italy Received 11 July 1980
Ultraviolet photoelectron spectroscopy (hi’ = 21. 2 eV) results on Si(III)/Au interfaces, prepared at liquid nitrogen temperature and at room temperature, are presented. It is shown for the first time that a considerable intermixing between Si and Au takes place also when the interface is prepared at liquid nitrogen temperature. This fact and the increase of the intermixing at increasing temperature (up to room temperature) are discussed and the role of the Au condensation energy in surface disruption and intermixing is pointed out.
The study of the disruption and intermixing mechanisms at work at semiconductor—metal interfaces is receiving increasing attention; in this connection a great amount of information has been obtained from photoemission. The study of the temperature dependence of interface intermixing is particularly significant and has been carried out extensively with photoemission at temperatures above RT on Si—Au interfaces [1,2] which are known to give very strong intermixing [3—51. The behaviour of the intermixing at low temperatures (from LNT to RT) has not yet been explored. Here we present an account of the first systematic work on this subject and we discuss the results in connection with the intermixing mechanism. The results presented here come from several sets of measurements taken on numerous samples with highly reproducible results. The interfaces were prepared by evaporating gold from a bead on a tungsten wire onto Si(III) faces cleaved in situ; a quartz oscillator was used to control the evaporation. The sample could be cooled down to 85 K (conventionally referred to as liquid nitrogen temperature, LNT) by circulating liquid nitrogen in the crystal holder and could be maintamed at higher temperatures by using indirect resistive heating. The gold bead was mounted at ~40 cm
from the sample to avoid heating of the sample during the evaporation. All photoelectron energy distribution curves (EDCs) were measured with Hel (21.2 eV) light by means of a spherical retarding potential spectrometer; the light was incident normally onto the sample and no magnetic perturbation on the spectrometer was ascertained due to the small current of the heater necessary to maintain the sample at temperatures between LNT and RT. An Auger spectrometer was also available in the vacuum chamber. All results refer to a coverage of 10 monolayers and thus refer to the metal-rich side of the interface; at these~coverageswe found that the EDCs are particularly s~sitiveto Si/Au intermixing. The EDCs from the interface prepared at LNT and measured successively at increasing temperature from LNT to RT are collected in fig. 1. We also give the EDC from an interface prepared at RT (10 mQ) measured a few minutes after the preparation and the EDC from 10 mQ of Au on an inert substrate (upper and lower dashed lines). The gold EDC is identical to that of bulk gold and within the noise does not depend on the nature of the substrate (we used clean Mo, oxidized Al, oxidized Cu and oxidized Mo) and on the temperature of the substrate (LNT or RT). The Au EDC is not influenced by the details of the growth mechanism as 69
Volume 80A, number 1
PHYSICS LETTERS
that the intermixing increases and that no Au agglomeration takes place. The sensitivity of the shallowest d-states to intermixing is discussed elsewhere [2] and is used here to introduce an empirical intermixing index a proportional to the decrease M of the area I of the EDCs in the region A indicated in fig. 2a. We define the index a = M/IAu where ‘Au is the value for the EDC of pure gold (the EDCs have been normalized to the same height of the shallowest peak). The values of a for increasing temperatures (at the indicated times after junction preparation) are plotted in fig. 2b where aRT is the value obtained immediately after preparation from a junction at RT. The initial EDC at LNT
______________________________
/ RI /‘ 400111
10 November 1980
‘,.
‘
P./~S/\~\~~
~3EPK
rize far andfrom the experiment herethe the final equilibrium; thepurpose main EDC while results are stationary which intermediate come onwork. the offrom EDCs this time the scale could preyisof be beyond ofthe thediscussion present Wepoint summaous analysis:
S
(i) the intermixing takes place also at LNT (although reduced with respect to RT); while (ii) the a junction same condition preparedisatreached LNT and in less heated thantoa RT few reaches the equilibrium on the time scale of an hour
10 ml Au
I
‘S
A -8
-6
—4
-~
_____________
-2 eV
Fig. 1. Angle integrated photoelectron energy distribution curves (EDCs) (hi’ = 21.2 eV) from Si(III) covered at LNT
with 10 monolayers of gold and successively heated to increasing temperatures for 15’ and held at RT for the times indicated (solid lines). The lower dashed EDC is that from the same quantity ofgold deposited onto an inert substrate. The upper dashed EDC was measured immediately after the deposition of 10 mQ of Au onto Si(III) held at RT.
--
__RT
-.
0.3
4 ~RT
expected from the localized nature of d-orbitals. The
difference at LNT between the Au EDC and the Si—Au EDC must be attributed to Si/Au intermixing. In fact the only way to influence drastically the d-density of states is to build some bonds between Au and Si. The intermixing at LNT is confirmed by the presence of a Si Auger signal at ~88.5 eV from the sample at LNT with a lineshape typical of Si bonded to Au [3]. The EDCs at increasing temperatures show an increasing departure from pure Au in the shallowest dregion while the Auger signal of combined Si increases and no Auger signal from free Si is seen. This means 70
a
0.1
______________
t=0 100
200
2 eV Ef 400 220 180 150’ (b) 300 °K
Fig. 2. (a) Comparison between the EDCs (hi’ = 21.2 eV) of 10 mQ of Au on an inert substrate (solid line) and the EDCs from two interfaces prepared with 10 m~of Au on Si(II1) held at LNT (dashed line) and at RT (dot-dashed line). The shaded area is the evidence of Si—Au intermixing at LNT. (b) The intermixing index (proportional to the shaded area) as a function of the temperature. The time after the preparation is given along the line; ‘~RTis the value typical of an interface prepared at RT.
Volume 80A, number I
PHYSICS LETTERS
minutes if the junction is prepared at RT. These points suggest that the thermal energy of the sample is not the only origin of the intermixing; some extra energy must be present during the junction preparation. This extra energy must cause the intermixing at LNT where thermally activated mass transport is strongly reduced. In fact the interplay between thermal energy and extra energy is essential to explain the above results: if both terms are present at RT (i.e. when the junction is preparedat RT) the equilibrium condition is reached very rapidly. If the equilibrium is to be reached at RT, after preparation at LNT, the bottleneck of mass transport can be overcome only by thermal agitation and a much longer time is required [result (ii)]. As far as the nature of this extra energy is concerned, a very reasonable assumption is that a relevant contribution is given by the condensation energy of the metal, i.e. by the energy which is released when a gold atom forms a bond to the surface. This term is surely more important than the kinetic energy of the incoming gold atoms in analogy to the explanation in
10 November 1980
refs. [6—8]where the role of the condensation energy during interface preparation with 111—V compounds was conjectured for the first time. Thus the present results are the first evidence of intermixing taking place at LNT in Si—Au interfaces and of the contribution of the condensation energy to the intermixing during the growth of semiconductor— metal interfaces. References [11 L. Braicovich et al., Phys. Rev. B20 (1979) 5131. [2] I. Abbati, L. Braicovich and A. Franciosi, Solid State Commun. 33 (1980) 881. [3] A.K. Green and E. Bauer, J. Appl. Phys. 47 (1976) 1284. [4] K. Oura and T. Hanawa, Surf. Sd. 82 (1979) 202. [5] K. Okuno, T. Ito, M. Iwami and A. Hiraki, Solid State Commun. 34 (1980) 493. [6] P.W. Chye, I. Lindau, P. Pianetta, C.M. Garner and W.E. Spicer, Phys. Rev. B17 (1978) 2682. [71 Lindau Vac. Rev. Sd. Technol. 15 5545. (1978) 1332. [81I. P.W. Chyeetetal.~ al.,J.Phys. B18 (1978)
71
PHYSICS LETTERS
10 November 1980
EVIDENCE OF INTERMIXING AT Si(llI)/Au INTERFACE AT LIQUID NITROGEN TEMPERATURE I. ABBATI and L. BRAICOVICH Istituto di Fisica del Politecnico di jltfilano, Milan, Italy
and A. FRANCIOSI 1stituto di Fisica deli’ Università di Roma, Rome, Italy Received 11 July 1980
Ultraviolet photoelectron spectroscopy (hi’ = 21. 2 eV) results on Si(III)/Au interfaces, prepared at liquid nitrogen temperature and at room temperature, are presented. It is shown for the first time that a considerable intermixing between Si and Au takes place also when the interface is prepared at liquid nitrogen temperature. This fact and the increase of the intermixing at increasing temperature (up to room temperature) are discussed and the role of the Au condensation energy in surface disruption and intermixing is pointed out.
The study of the disruption and intermixing mechanisms at work at semiconductor—metal interfaces is receiving increasing attention; in this connection a great amount of information has been obtained from photoemission. The study of the temperature dependence of interface intermixing is particularly significant and has been carried out extensively with photoemission at temperatures above RT on Si—Au interfaces [1,2] which are known to give very strong intermixing [3—51. The behaviour of the intermixing at low temperatures (from LNT to RT) has not yet been explored. Here we present an account of the first systematic work on this subject and we discuss the results in connection with the intermixing mechanism. The results presented here come from several sets of measurements taken on numerous samples with highly reproducible results. The interfaces were prepared by evaporating gold from a bead on a tungsten wire onto Si(III) faces cleaved in situ; a quartz oscillator was used to control the evaporation. The sample could be cooled down to 85 K (conventionally referred to as liquid nitrogen temperature, LNT) by circulating liquid nitrogen in the crystal holder and could be maintamed at higher temperatures by using indirect resistive heating. The gold bead was mounted at ~40 cm
from the sample to avoid heating of the sample during the evaporation. All photoelectron energy distribution curves (EDCs) were measured with Hel (21.2 eV) light by means of a spherical retarding potential spectrometer; the light was incident normally onto the sample and no magnetic perturbation on the spectrometer was ascertained due to the small current of the heater necessary to maintain the sample at temperatures between LNT and RT. An Auger spectrometer was also available in the vacuum chamber. All results refer to a coverage of 10 monolayers and thus refer to the metal-rich side of the interface; at these~coverageswe found that the EDCs are particularly s~sitiveto Si/Au intermixing. The EDCs from the interface prepared at LNT and measured successively at increasing temperature from LNT to RT are collected in fig. 1. We also give the EDC from an interface prepared at RT (10 mQ) measured a few minutes after the preparation and the EDC from 10 mQ of Au on an inert substrate (upper and lower dashed lines). The gold EDC is identical to that of bulk gold and within the noise does not depend on the nature of the substrate (we used clean Mo, oxidized Al, oxidized Cu and oxidized Mo) and on the temperature of the substrate (LNT or RT). The Au EDC is not influenced by the details of the growth mechanism as 69
Volume 80A, number 1
PHYSICS LETTERS
that the intermixing increases and that no Au agglomeration takes place. The sensitivity of the shallowest d-states to intermixing is discussed elsewhere [2] and is used here to introduce an empirical intermixing index a proportional to the decrease M of the area I of the EDCs in the region A indicated in fig. 2a. We define the index a = M/IAu where ‘Au is the value for the EDC of pure gold (the EDCs have been normalized to the same height of the shallowest peak). The values of a for increasing temperatures (at the indicated times after junction preparation) are plotted in fig. 2b where aRT is the value obtained immediately after preparation from a junction at RT. The initial EDC at LNT
______________________________
/ RI /‘ 400111
10 November 1980
‘,.
‘
P./~S/\~\~~
~3EPK
rize far andfrom the experiment herethe the final equilibrium; thepurpose main EDC while results are stationary which intermediate come onwork. the offrom EDCs this time the scale could preyisof be beyond ofthe thediscussion present Wepoint summaous analysis:
S
(i) the intermixing takes place also at LNT (although reduced with respect to RT); while (ii) the a junction same condition preparedisatreached LNT and in less heated thantoa RT few reaches the equilibrium on the time scale of an hour
10 ml Au
I
‘S
A -8
-6
—4
-~
_____________
-2 eV
Fig. 1. Angle integrated photoelectron energy distribution curves (EDCs) (hi’ = 21.2 eV) from Si(III) covered at LNT
with 10 monolayers of gold and successively heated to increasing temperatures for 15’ and held at RT for the times indicated (solid lines). The lower dashed EDC is that from the same quantity ofgold deposited onto an inert substrate. The upper dashed EDC was measured immediately after the deposition of 10 mQ of Au onto Si(III) held at RT.
--
__RT
-.
0.3
4 ~RT
expected from the localized nature of d-orbitals. The
difference at LNT between the Au EDC and the Si—Au EDC must be attributed to Si/Au intermixing. In fact the only way to influence drastically the d-density of states is to build some bonds between Au and Si. The intermixing at LNT is confirmed by the presence of a Si Auger signal at ~88.5 eV from the sample at LNT with a lineshape typical of Si bonded to Au [3]. The EDCs at increasing temperatures show an increasing departure from pure Au in the shallowest dregion while the Auger signal of combined Si increases and no Auger signal from free Si is seen. This means 70
a
0.1
______________
t=0 100
200
2 eV Ef 400 220 180 150’ (b) 300 °K
Fig. 2. (a) Comparison between the EDCs (hi’ = 21.2 eV) of 10 mQ of Au on an inert substrate (solid line) and the EDCs from two interfaces prepared with 10 m~of Au on Si(II1) held at LNT (dashed line) and at RT (dot-dashed line). The shaded area is the evidence of Si—Au intermixing at LNT. (b) The intermixing index (proportional to the shaded area) as a function of the temperature. The time after the preparation is given along the line; ‘~RTis the value typical of an interface prepared at RT.
Volume 80A, number I
PHYSICS LETTERS
minutes if the junction is prepared at RT. These points suggest that the thermal energy of the sample is not the only origin of the intermixing; some extra energy must be present during the junction preparation. This extra energy must cause the intermixing at LNT where thermally activated mass transport is strongly reduced. In fact the interplay between thermal energy and extra energy is essential to explain the above results: if both terms are present at RT (i.e. when the junction is preparedat RT) the equilibrium condition is reached very rapidly. If the equilibrium is to be reached at RT, after preparation at LNT, the bottleneck of mass transport can be overcome only by thermal agitation and a much longer time is required [result (ii)]. As far as the nature of this extra energy is concerned, a very reasonable assumption is that a relevant contribution is given by the condensation energy of the metal, i.e. by the energy which is released when a gold atom forms a bond to the surface. This term is surely more important than the kinetic energy of the incoming gold atoms in analogy to the explanation in
10 November 1980
refs. [6—8]where the role of the condensation energy during interface preparation with 111—V compounds was conjectured for the first time. Thus the present results are the first evidence of intermixing taking place at LNT in Si—Au interfaces and of the contribution of the condensation energy to the intermixing during the growth of semiconductor— metal interfaces. References [11 L. Braicovich et al., Phys. Rev. B20 (1979) 5131. [2] I. Abbati, L. Braicovich and A. Franciosi, Solid State Commun. 33 (1980) 881. [3] A.K. Green and E. Bauer, J. Appl. Phys. 47 (1976) 1284. [4] K. Oura and T. Hanawa, Surf. Sd. 82 (1979) 202. [5] K. Okuno, T. Ito, M. Iwami and A. Hiraki, Solid State Commun. 34 (1980) 493. [6] P.W. Chye, I. Lindau, P. Pianetta, C.M. Garner and W.E. Spicer, Phys. Rev. B17 (1978) 2682. [71 Lindau Vac. Rev. Sd. Technol. 15 5545. (1978) 1332. [81I. P.W. Chyeetetal.~ al.,J.Phys. B18 (1978)
71