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The adsorption of tin layers on cleaved indium phosphide surfaces
Surface Science 152/153 (1985) 1222-1227 North-Holland, Amsterdam
1222
THE ADSORPTION OF TIN LAYERS ON CLEAVED INDIUM PHOSPHIDE SURFACES T.P. HUMPHREYS, E.C. CUNNINGHAM Physrcs Department,
Received
G.J. HUGHES *, A. MCKINLEY, and R.H. WILLIAMS
University College, Cardiff, UK
2 April 1984
The growth of tin overlayers on vacuum cleaved (110) surfaces of indium phosphide has been studied using a multi-technique approach. The growth mode appears both coverage and temperature dependent. The systematics of the adsorption process as well as the mechanism driving the interdiffusion of atoms across the interface are considered as well as the influence of the interdiffusion on the electrical nature of the interface.
1. Introduction The formation of good electrical constants to semiconductors is of great technological importance, yet our understanding of the fundamental physical mechanisms governing such contacts is very far from complete. Many recent studies [l--4] aimed at understanding metal-semiconductor interfaces have involved the application of modern surface science techniques to probe the interaction of thin metal overlayers with atomically clean cleaved semiconductor surfaces and have resulted in new models of Schottky barrier formation, for example. Chemical reactions and intermixing occur commonly at these interfaces and the influence of these on the electrical nature of the contact is an important question. In this paper we consider the nature of very thin tin overlayers on an atomically clean III-V semiconductor surface, namely the vacuum cleaved InP(ll0) surface. This interface is a particularly interesting one because group IV atoms can form donors or acceptors in III-V semiconductors and any indiffusion of Sn atoms may be expected to have a pronounced effect on the electrical properties should the Sn atoms adopt substitutional sites. Alloy contacts to III-V semiconductors often include group IV atoms when ohmic behaviour is required but the precise role of these atoms is not fully understood. * Now at IBM, Yorktown
Heights,
New York 10598, USA.
0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
T.P. Humphreys
et al. / Tin layers on cleaved InP
1223
2. Experimental Several ultra high vacuum systems were used for these studies [5]. All contained facilities for cleaving samples in UHV and LEED, Auger spectroscopy and photoluminescence techniques were available as well as means of measuring diode characteristics in situ on interfaces prepared in ultra high vacuum. Some studies were carried out in an angle resolving photoelectron spectrometer attached to the synchrotron storage ring (SRS) at Daresbury laboratory. The InP crystals were all n-type with carrier concentration of - 5 x lOI -3. they were cleaved in vacuum to give high quality surfaces onto which Sn cm films were deposited by evaporation from tungsten filaments. Samples could be cooled to 100 K during deposition of Sn in some of the systems. The distance to the filament was large enough to ensure no increase in temperature of the sample.
3. Results and discussion Fig. 1 shows the In 4d and P 2p core level photoemission peaks obtained using the synchrotron source. As tin is deposited on the clean InP(llO) surface
16
18 BE id)
BE@/)
Fig. 1. Photoemission intensity spectra for the Sn 4d, the indium 4d and P 2p core levels as a function of Sn thickness deposited onto the InP(110) surface. Photon energy is 100 eV for the Sn and In peaks and 170 eV for the P peak. Thickness of Sn are shown in A on the curves.
1224
T.P. Humphreys
et al. / Tin layers on cleaved InP
the In 4d and P 2p emissions are attenuated whereas the Sn 4d emission increases. The Sn 4d emission is broad at low coverages and the spin-orbit split components are not well defined until the coverage exceeds around 3 A. In addition the binding energy decreases with increasing coverage for the Sn 4d emission. This behaviour is quite often observed for adsorbates on semiconductors and almost certainly reflects chemical interactions between the Sn adlayer and the InP(I 10) surface at low coverages. The emission corresponding to 12 A coverage largely reflects the thick Sn metal film. The In 4d and P 2p emissions are also shifted somewhat with increasing Sn coverage. Initially both are shifted to lower binding energy, by a small amount, and as the coverage increases the In 4d emission shifts further to lower energies. The attenuation of the core levels as a function of Sn coverage are plotted in fig. 2. Initially the attenuation is exponential and is equal for In 4d and P 2p emission, entirely consistent with layer by layer growth. Beyond about 3 A, however, the attenuation deviates from the exponential and the In 4d emission decreases more slowsly than the P 2p case. These attenuation rates reflect the well known Stranski-Krastanov [6] growth mechanism, i.e., uniform layer growth initially followed by island growth at larger coverages. At the higher coverages there also appears to be some degree of intermixing of the
0
20
10
THICKNESS
30
6,
Fig. 2. Attenuation of the In 4d and P 2p core level photoemission as a function of coverage at room temperature. Curves A are for Ga overlayers, curves B for Sn layers, and curve C for Sb layers, all on InP(llO) clean cleaved surfaces. The dotted line corresponds to an exponential attenuation with mean free path of 4 A.
T.P. Humphreys et al. / Tin layers on cleaved InP
1225
atomic constituents across the boundary, with In metal being released and incorporated in the growing Sn overlayer. In fig. 2 we also show similar attenuation plots for Ga (group III) and Sb (group V) adsorption on the III-V solid InP [7]. Examination of the deposited thick Sn film with the scanning electron microscope revealed that films deposited at room temperature are indeed composed of large islands. However, films deposited at 100 K appeared smooth and featureless. Transport studies carried out on Sn films deposited onto clean InP(110) surfaces at 100 K showed the formation of Schottky barriers at the Sn: InP interface. This barrier exists even when the temperature of the diode is as illustrated in fig. 3. In many of the increased to room temperature, situations where Sn was deposited at room temperature, however, the Sn : InP contacts were found to be ohmic. This is also illustrated in fig. 3. Interestingly, Sn films deposited on etched InP(110) surfaces also reveal rather leaky Schottky barriers which become highly ohmic when the contact is laser annealed. TO summarise, therefore, it appears that the growth of Sn layers on clean cleaved InP(110) surfaces is dependent both on the layer thickness and on the
Fig. 3. I-V characteristics measured for a Sn contact on a cleaved InP(llO) surfaces. Curve A is for a Sn contact deposited at room temperature while curve B was obtained for a Sn contact deposited at 77 K but measured at room temperature.
T.P. Humphreys
1226
Sn
et al. /
Tin lqvers on cleaved InP
Minsk” . . . . . . ‘1n.P’ . - . - . . . . . . . . . . . . . . . . .
.
Stranski-Krastanov Layer + Islands INTERMIXING
Sn 777777777777727;
.
.
.
.
.
.
. .
.
. .
‘.
*InP
* .
.
.
.
>
. .
.
Low temperature Thin layers
.
.
.
Sn doping n+ Layer Thin barrier Ohmic contact
. .
*
.
. .
L
.
.
layer Layer grow7 h
Schottky
barriers
Fig. 4. Schematic representation of the electrical behaviour obtained for the deposition of Sn on InP(110) at room and liquid nitrogen temperatures. The growth modes for the Sn overlayers are also represented schematically.
temperature of the surface during deposition. At room temperature the growth appears Lo follow the Stranski-Krastanov mechanism of layer followed by islands. When islands do form intermixing also seems to occur at the interface and there are strong indications that the energy released in the formation of nuclei drives the interfacial intermixing [4,8]. The shifts of photoemission core levels indicate that a Schottky barrier is formed for deposition of 1 or 2 A of Sn at room temperature. The ohmic contacts clearly appear to be associated with the intermixing which occurs at higher coverages. The natural explanation is that the InP(110) surface is doped highly ni and that the electrical barrier is so thin that electrons simply tunnel through it, yielding ohmic behaviour. Laser annealing causes the indiffusion of Sn and also leads to n+ ohmic contacts. The mechanisms are illustrated in fig. 4 whereby the kinetically driven intermixing leads to ni surface layers and ohmic behaviour. As further evidence of this we investigated the influence of thin Sn adlayers at interfaces between InP(ll0) surfaces and Ag contacts. Intimate Ag: InP(110) contacts yield
T.P. Humphreys et al. / Tin layers on cleaved InP
1227
Schottky barriers of - 0.5 eV. Thickness of 1 to 2 A of Sn deposited at room temperature on the InP surface, followed by the deposition of thick Ag layers, to the linear region in fig. 2, also yield barriers of - 0.5 eV. This corresponds where no intermixing of Sn and InP is observed. If the Sn layer is thicker than - 3 A, however, then the Ag(Sn)InP interface becomes ohmic due to the intermixing.
Acknowledgements
We wish to thank DCVD, SERC and British Telecom We also wish to thank Drs. I.T. McGovern, D. Norman their support.
for financial support. and H. Padmore for
References [l] W.E. Spicer, I. Lindau, P.R. Skeath, C.Y. Su and P.W. Chye, Phys. Rev. Letters 44 (1980) 420. [2] L.J. Brillson, C.F. Brucker, A.D. Katnani, N.G. Stoffel, R. Daniels and C. Margaritondo, Surface Sci. 132 (1983) 212. [3] R.H. Williams, Contemporary Phys. 23 (1982) 329. [4] R. Ludeke and G. Landgren, J. Vacuum Sci. Technol. 19 (1981) 667. [S] A. McKinley, G.J. Hughes and R.H. Williams, J. Phys. Cl5 (1982) 7049. [6] R. Kern, G. LeLay and J.J. Metois, in: Current Topics in Materials Science, Vol. 3, Ed. E. Kaldis (North-Holland, Amsterdam, 1979) ch. 3. (71 R.H. Williams, A. McKinley, G.J. Hughes, C. Maani and T.P. Humphreys, J. Vacuum Sci. Technol., in press. [S] A. Zunger, Phys. Rev. B24 (1981) 4372.
1222
THE ADSORPTION OF TIN LAYERS ON CLEAVED INDIUM PHOSPHIDE SURFACES T.P. HUMPHREYS, E.C. CUNNINGHAM Physrcs Department,
Received
G.J. HUGHES *, A. MCKINLEY, and R.H. WILLIAMS
University College, Cardiff, UK
2 April 1984
The growth of tin overlayers on vacuum cleaved (110) surfaces of indium phosphide has been studied using a multi-technique approach. The growth mode appears both coverage and temperature dependent. The systematics of the adsorption process as well as the mechanism driving the interdiffusion of atoms across the interface are considered as well as the influence of the interdiffusion on the electrical nature of the interface.
1. Introduction The formation of good electrical constants to semiconductors is of great technological importance, yet our understanding of the fundamental physical mechanisms governing such contacts is very far from complete. Many recent studies [l--4] aimed at understanding metal-semiconductor interfaces have involved the application of modern surface science techniques to probe the interaction of thin metal overlayers with atomically clean cleaved semiconductor surfaces and have resulted in new models of Schottky barrier formation, for example. Chemical reactions and intermixing occur commonly at these interfaces and the influence of these on the electrical nature of the contact is an important question. In this paper we consider the nature of very thin tin overlayers on an atomically clean III-V semiconductor surface, namely the vacuum cleaved InP(ll0) surface. This interface is a particularly interesting one because group IV atoms can form donors or acceptors in III-V semiconductors and any indiffusion of Sn atoms may be expected to have a pronounced effect on the electrical properties should the Sn atoms adopt substitutional sites. Alloy contacts to III-V semiconductors often include group IV atoms when ohmic behaviour is required but the precise role of these atoms is not fully understood. * Now at IBM, Yorktown
Heights,
New York 10598, USA.
0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
T.P. Humphreys
et al. / Tin layers on cleaved InP
1223
2. Experimental Several ultra high vacuum systems were used for these studies [5]. All contained facilities for cleaving samples in UHV and LEED, Auger spectroscopy and photoluminescence techniques were available as well as means of measuring diode characteristics in situ on interfaces prepared in ultra high vacuum. Some studies were carried out in an angle resolving photoelectron spectrometer attached to the synchrotron storage ring (SRS) at Daresbury laboratory. The InP crystals were all n-type with carrier concentration of - 5 x lOI -3. they were cleaved in vacuum to give high quality surfaces onto which Sn cm films were deposited by evaporation from tungsten filaments. Samples could be cooled to 100 K during deposition of Sn in some of the systems. The distance to the filament was large enough to ensure no increase in temperature of the sample.
3. Results and discussion Fig. 1 shows the In 4d and P 2p core level photoemission peaks obtained using the synchrotron source. As tin is deposited on the clean InP(llO) surface
16
18 BE id)
BE@/)
Fig. 1. Photoemission intensity spectra for the Sn 4d, the indium 4d and P 2p core levels as a function of Sn thickness deposited onto the InP(110) surface. Photon energy is 100 eV for the Sn and In peaks and 170 eV for the P peak. Thickness of Sn are shown in A on the curves.
1224
T.P. Humphreys
et al. / Tin layers on cleaved InP
the In 4d and P 2p emissions are attenuated whereas the Sn 4d emission increases. The Sn 4d emission is broad at low coverages and the spin-orbit split components are not well defined until the coverage exceeds around 3 A. In addition the binding energy decreases with increasing coverage for the Sn 4d emission. This behaviour is quite often observed for adsorbates on semiconductors and almost certainly reflects chemical interactions between the Sn adlayer and the InP(I 10) surface at low coverages. The emission corresponding to 12 A coverage largely reflects the thick Sn metal film. The In 4d and P 2p emissions are also shifted somewhat with increasing Sn coverage. Initially both are shifted to lower binding energy, by a small amount, and as the coverage increases the In 4d emission shifts further to lower energies. The attenuation of the core levels as a function of Sn coverage are plotted in fig. 2. Initially the attenuation is exponential and is equal for In 4d and P 2p emission, entirely consistent with layer by layer growth. Beyond about 3 A, however, the attenuation deviates from the exponential and the In 4d emission decreases more slowsly than the P 2p case. These attenuation rates reflect the well known Stranski-Krastanov [6] growth mechanism, i.e., uniform layer growth initially followed by island growth at larger coverages. At the higher coverages there also appears to be some degree of intermixing of the
0
20
10
THICKNESS
30
6,
Fig. 2. Attenuation of the In 4d and P 2p core level photoemission as a function of coverage at room temperature. Curves A are for Ga overlayers, curves B for Sn layers, and curve C for Sb layers, all on InP(llO) clean cleaved surfaces. The dotted line corresponds to an exponential attenuation with mean free path of 4 A.
T.P. Humphreys et al. / Tin layers on cleaved InP
1225
atomic constituents across the boundary, with In metal being released and incorporated in the growing Sn overlayer. In fig. 2 we also show similar attenuation plots for Ga (group III) and Sb (group V) adsorption on the III-V solid InP [7]. Examination of the deposited thick Sn film with the scanning electron microscope revealed that films deposited at room temperature are indeed composed of large islands. However, films deposited at 100 K appeared smooth and featureless. Transport studies carried out on Sn films deposited onto clean InP(110) surfaces at 100 K showed the formation of Schottky barriers at the Sn: InP interface. This barrier exists even when the temperature of the diode is as illustrated in fig. 3. In many of the increased to room temperature, situations where Sn was deposited at room temperature, however, the Sn : InP contacts were found to be ohmic. This is also illustrated in fig. 3. Interestingly, Sn films deposited on etched InP(110) surfaces also reveal rather leaky Schottky barriers which become highly ohmic when the contact is laser annealed. TO summarise, therefore, it appears that the growth of Sn layers on clean cleaved InP(110) surfaces is dependent both on the layer thickness and on the
Fig. 3. I-V characteristics measured for a Sn contact on a cleaved InP(llO) surfaces. Curve A is for a Sn contact deposited at room temperature while curve B was obtained for a Sn contact deposited at 77 K but measured at room temperature.
T.P. Humphreys
1226
Sn
et al. /
Tin lqvers on cleaved InP
Minsk” . . . . . . ‘1n.P’ . - . - . . . . . . . . . . . . . . . . .
.
Stranski-Krastanov Layer + Islands INTERMIXING
Sn 777777777777727;
.
.
.
.
.
.
. .
.
. .
‘.
*InP
* .
.
.
.
>
. .
.
Low temperature Thin layers
.
.
.
Sn doping n+ Layer Thin barrier Ohmic contact
. .
*
.
. .
L
.
.
layer Layer grow7 h
Schottky
barriers
Fig. 4. Schematic representation of the electrical behaviour obtained for the deposition of Sn on InP(110) at room and liquid nitrogen temperatures. The growth modes for the Sn overlayers are also represented schematically.
temperature of the surface during deposition. At room temperature the growth appears Lo follow the Stranski-Krastanov mechanism of layer followed by islands. When islands do form intermixing also seems to occur at the interface and there are strong indications that the energy released in the formation of nuclei drives the interfacial intermixing [4,8]. The shifts of photoemission core levels indicate that a Schottky barrier is formed for deposition of 1 or 2 A of Sn at room temperature. The ohmic contacts clearly appear to be associated with the intermixing which occurs at higher coverages. The natural explanation is that the InP(110) surface is doped highly ni and that the electrical barrier is so thin that electrons simply tunnel through it, yielding ohmic behaviour. Laser annealing causes the indiffusion of Sn and also leads to n+ ohmic contacts. The mechanisms are illustrated in fig. 4 whereby the kinetically driven intermixing leads to ni surface layers and ohmic behaviour. As further evidence of this we investigated the influence of thin Sn adlayers at interfaces between InP(ll0) surfaces and Ag contacts. Intimate Ag: InP(110) contacts yield
T.P. Humphreys et al. / Tin layers on cleaved InP
1227
Schottky barriers of - 0.5 eV. Thickness of 1 to 2 A of Sn deposited at room temperature on the InP surface, followed by the deposition of thick Ag layers, to the linear region in fig. 2, also yield barriers of - 0.5 eV. This corresponds where no intermixing of Sn and InP is observed. If the Sn layer is thicker than - 3 A, however, then the Ag(Sn)InP interface becomes ohmic due to the intermixing.
Acknowledgements
We wish to thank DCVD, SERC and British Telecom We also wish to thank Drs. I.T. McGovern, D. Norman their support.
for financial support. and H. Padmore for
References [l] W.E. Spicer, I. Lindau, P.R. Skeath, C.Y. Su and P.W. Chye, Phys. Rev. Letters 44 (1980) 420. [2] L.J. Brillson, C.F. Brucker, A.D. Katnani, N.G. Stoffel, R. Daniels and C. Margaritondo, Surface Sci. 132 (1983) 212. [3] R.H. Williams, Contemporary Phys. 23 (1982) 329. [4] R. Ludeke and G. Landgren, J. Vacuum Sci. Technol. 19 (1981) 667. [S] A. McKinley, G.J. Hughes and R.H. Williams, J. Phys. Cl5 (1982) 7049. [6] R. Kern, G. LeLay and J.J. Metois, in: Current Topics in Materials Science, Vol. 3, Ed. E. Kaldis (North-Holland, Amsterdam, 1979) ch. 3. (71 R.H. Williams, A. McKinley, G.J. Hughes, C. Maani and T.P. Humphreys, J. Vacuum Sci. Technol., in press. [S] A. Zunger, Phys. Rev. B24 (1981) 4372.