- Email: [email protected]
Carbon Auger peak shape measurements in the characterization of reactions on (001) diamond
__ __ EB
&
applied
surface science
EL/SEWER
Applied Surface Science lOO/lOl
(1996) 612-616
Carbon Auger peak shape measurements in the characterization reactions on (001) diamond
of
P.E. Viljoen a3*, W.D. Roos a, H.C. Swart a, P.H. Holloway b a Department ofPhysics, ’ Department
of Materials
UOFS. P.O. Box 339. Bloemfontein
Science and Engineerin,g.
Received 22 August
Unirersi&
1995; accepted
of Florida,
17 November
9300. South Africa Gainewille.
FL 32611.
USA
1995
Abstract The measurements of Auger peak shape changes and peak shifts as a fingerprint for chemical changes on solid surfaces are well known. The carbon Auger peak shape changes have been measured to characterize reactions on a (001) diamond single crystal surface. Firstly the influence of Ar+ ion sputtering on the diamond surface was investigated. Upon sputtering the surface, the carbon Auger peak shape changes from the typical diamond to the graphite form within about one minute sputtering. The peak form change with time has been correlated with different diamond and graphite contributions. The decrease of the diamond contribution with sputter time has been described in terms of a transition probability of the carbon sp3 to sp’ bond. Secondly the formation of TIC at the interface of a Ti overlayer and the diamond substrate upon annealing, in order to produce good and stable ohmic contacts to the diamond, was confirmed by comparing the measured data with data simulated by calculation of the carbide and graphite contributions to the final carbon (KLL) peak shape.
1. Introduction In the last three
decades
the carbon
KLL
Auger
has been measured and extensively used to determine the chemical state of carbon on solid surfaces. The Auger Handbook [1] gives the most prominent shapes, e.g. carbide and graphite although other shapes have been reported in the literature. Of the earlier pioneer work is well cited in the literature. The influence of sputtering on the peak shape of carbon was investigated by Koscher et al. [2] and Bellina and Zeller [3]. The carbon (KLL) peak shape changes of diamond films have been studied by peak shape
Ferrer et al. [4] and Ferrari et al. [S]. Most of the work mentioned were of a qualitative nature. Only in a limited number of cases have quantitative methods been used. In this study the carbon (KLL) Auger peak shape have been measured on the (001) surface of diamond and quantitative methods have been used to describe the observed spectra in the characterization of reactions on the surface.
2. Experiment
The full experimental details were described elsewhere [6] and only a brief description will be given
’ Corresponding author. E-mail: [email protected]. 0 169.4332/96/$15.00 Copyright PII SO169-4332(96)00349-2
here. The single
0 1996 Elsevier Science B.V. All rights reserved
crystal
diamond,
equivalent
to a
P.E. Viljoen et al/Applied
Surfiice Science lOO/ 101 (1996) 612-616
class IIb natural diamond, was a boron doped p-type semiconductor. It was synthetically grown by a high high pressure technique [7]. It was temperature. kindly supplied by the De Beers of South Africa. The orientation was done with the back reflection Laue method. The two larger flat surfaces of the crystal, in the form of a matchbox, are parallel to the (001) crystallographic plane. Measurements were done on these surfaces. Both the Ti and Au layers were sputter deposited in a diffusion pumped vacuum system with a base pressure of about 8 X lop7 Torr. A USGun planar magnetron with a 5 cm diameter target at 20 mTorr Ar pressure, was used. The deposition rate was about 0.4 rim/s.. The film thicknesses were determined by a Sloan Dektak profilometer and checked by calculations from RBS measurements. Auger measurements were made in a PHI model 545 UHV system using a single pass CMA with the electron gun operated at 1 PA and 5 keV. In this system the sample was sputtered with 2 keV Ar+ ions by backfilling the system with pure Ar gas to 5 X 10e5 Torr. Auger spectra and depth profiles were also measured with a Perkin-Elmer PHI model 660 SAM using 10 keV electrons with a 50 nA beam current. In this case the sputtering was done with 3 keV Arf ions from a differentially pumped ion gun incident on the target at about 45” and also rastered over the surface. Annealing of the sample was done in a tube furnace under an atmosphere of 99.99% pure forming gas consisting of 90% N, and 10% HZ.
3. Results and discussion Fig. 1 shows the carbon KLL Auger peak for the diamond (001) surface for sputtering times from 0 to 60 s as indicated. The change from the typical diamond shape for no sputtering to the graphite shape after 60 s sputtering is evident. The Ar peak at 215 eV increases with sputtering time. Great care has been taken to ensure that the peak shape change was due to phase changes in the diamond and not to adsorbed species. The diamond peak shape was monitored and did not change even after two weeks in the UHV system during which time several Ar gas flushes and sputtering of the samples holder took
613
G?
.z 3
e J g 2
W
z 200
C(KLL)
220
Auger
peak
240
260
280
300
320
Electron energy (eV) Fig. 1. The measured carbon KLL Auger peak for the diamond (001) surface at sputtering times as indicated. The surface was sputtered with 2 keV Ar + ions rastered over the surface.
place. Under sputtering of the diamond surface the peak changes within seconds. This is in accordance with other observations. In order to quantify the results, standard spectra for diamond and graphite were obtained. The diamond peak was measured on a clean unsputtered diamond surface. Only a very small amount of oxygen was present on the surface. The graphite peak shape was obtained from the Auger Handbook [l]. The relative peak heights of the diamond and graphite were established by measuring the pure diamond peak before and after extensive sputtering to make sure that the surface was totally graphitized. The standards are shown in Fig. 2. The respective contributions of diamond and graphite to a specific peak in Fig. 1, was done by a Fortran program which makes use of the E04CCF NAG routine to minimize the function F(X) with N dependents X = (X,, X,, .. .. X,) using the Simplex method. It is an iterative process that does not need any differentials of the function. The value of the given function is determined in the fit as the sum of difference squared e.g.
(1) where M(X) is the measured spectrum and D(X) and G(X) respectively the diamond and graphite standards at the specific X or energy value. The fitting parameters N, and N2 are then determined by
614
P.E. Viljoen et al./Applied
-
Standards
-
- -
Surface Science 100 / 101 (19961612-616
Diamond Graphite
Diamond 0.8 i\
220
200
280
260
240
300
.
20
Electron energy (eV)
an iterative process. The sum of these two parameters for a binary alloy should be equal to 1.0 or the percentage contributions of diamond and graphite should add up to 100%. A fit has been made to all the spectra of Fig. 1. A couple of these are shown in Fig. 3(a) and (b) indicating the measured and fitted curves. The percentages in all cases add up to about 100%. These fits are all fairly good and the percentage diamond contribution to the measured peak, as determined from the fitted data, is shown in Fig. 4 as a function
-
---
(4
Measured Fitted
_ Sputtertime -
-
= 0 sec. 82% diamood:17% graphite
Sputter time = 20 see. dismond:66% graphite
_ 32%
I 200
220
’
I 240
’
1
’
260
40
Sputter
Fig. 2. The standard carbon KLL Auger spectra for the diamond and graphite. The diamond peak was measured on a clean unsputtered diamond surface. The graphite peak was taken from Ref. 111.
I 280
’ 300
Electron energy (ev) Fig. 3. The measured and fitted curves for the carbon KLL Auger peak for the diamond (001) surface at sputtering times as indicated in (a) and (b). The surface was sputtered with 2 keV Ar+ ions rastered over the surface. The fitted curves were calculated with F.q (1) by using the standard spectra.
(001)
’
60
80
100
120
140
time (seconds)
Fig. 4. The calculated diamond contribution to the carbon KLL Auger spectra as a function of the sputtering time. The solid line is a non-linear least squares fit of Eq. (2) to the data. The exponential decrease with sputter time is evident.
of the sputter time. Since some graphitization already exist for zero sputter time (see Fig. 3), the time in Fig. 4 has been extended back by 10.0 s to include the 100% diamond contribution point. The exponential decrease in the diamond contribution upon sputtering is quite evident as shown by the solid line. The solid line in Fig. 4 was calculated from the following equation [8]: Z=I,exp(
-air)
(2)
where I is the signal from the sp3 carbon bond or diamond Auger peak shape, cr the transformation probability of the sp3 to the sp’ carbon bond, i is the ion current density and the product i ’ r is the fluence of Ar ions onto the surface. According to Taglauer [8] Eq. (2) comes from the fact that the number of diamond (or sp3 bonding) carbon atoms transformed into graphite (sp’ bonding) at a rate that is proportional to the number (or fluence it) of ions hitting the surface, that is the sputtering time. From a fit of this equation to the data by a non-linear least squares method, as shown in Fig. 4, a value of ui is determined as 0.027 18 s-‘. The ion current density i was 50 X 10-l A m-’ or 3.125 X 10” ions sP ’ rn-‘. One can express the ion current density in terms of the number of ions impinging per surface atom. The density of surface atoms on the (001) diamond surface is 1.58 X lOI m-’ [9]. Thus i = 0.2 ions per surface atom per second. This gives
P.E. Vi&en et al./Applied
Stfuce
a value for u of 0.14 transformations per incoming ion. This is an estimated value which is of the right order of magnitude of what one would expect for an ion beam of this nature. Ohmic contact to the (001) surface of this diamond was established by sputter deposition of a 200 nm titanium layer followed by a 100 nm gold layer on top of the Ti to prevent oxidation during the annealing and handling process. The sample was then annealed for different times at different temperatures [6]. Very good ohmic contact was achieved by annealing the sample at 750°C for about 10 min or more in an inert atmosphere of 99.99% pure forming gas. A depth profile, Fig. 5, of the sample after annealing at 750°C for 60 min, shows amongst other things that, at the diamond/titanium interface, the KLL carbon Auger peak shape changes from a carbide rich to a graphite form as one sputters through the interface. This is clearly shown by Fig. 6 which indicate complete Auger spectra of the sample at sputtering times, as indicated, see also Fig. 5. The fact that the depth profile does not indicate a sharp interface is due to the unevenness of the surface as explained elsewhere [6]. The inserts in Fig. 6 next to the carbon peaks in (al and (b), are carbon peaks simulated by calculating linear combinations of carbide and graphite con-
0
0
20
40
60
80
100
120
140
Sputter time (min) Fig. 5. Depth profile for a Au on Ti on diamond (001) sample after annealing at 750°C for 60 min. In this case the Ti and Au layers were respectively 200 and 250 nm thick. Ti(1) and Tic?) are respectively the Tic387 eV) and the Tic418 eV) Auger peaks. Sputtering was done by a differentially pumped ion gun with 3 keV Ar+ ions rastered over the surface.
Science lOO/lOl(19951612-616
615
(h)
(
1 (c)
0
. 0
, . , . , . , . , . , . 100 200 300 400 500 600 700
Electron energy (eV) Fig. 6. Auger spectra from Au on Ti on diamond (001) sample annealed at 750°C for 60 min after sputtering times of (a) 100. (b) 120, and (c) 140 min (see Fig. 5). The insets in (a) and (b) are the simulated C peaks for carbide contributions to the peak of 50% and 10%. respectively. The carbon peak in (c) is from graphite, which results from the continuous ion bombardment of diamond surfaces.
tributions to the final peak shape. The carbide peak shape which has been used as standard, was taken from the Auger Handbook [l] for TaC which is a very typical carbide shape. The relative peak heights were determined from the peak heights and amplification factors in the PHI Auger Handbook [l]. The calculated and measured peak shapes are very similar and the percentage contribution can be considered as fairly accurate. It clearly shows the existence of carbide at the interface which, upon sputtering into the substrate, disappear and only the sputter converted graphite form of the diamond peak remains. Calculation with Eq. (1) could be performed in this case, however, it becomes much more difficult due to the larger number of elements present. Since the result in this case is more qualitative in nature, we only used the linear combinations. The existence of carbide at the interface indicates the formation of TIC which is a prerequisite for good ohmic contacts. This was always suggested but the proof for that is known only in a few cases. This is also in agreement with RBS measurements [6,10].
616
P.E. Viljoert et &./Applied
SurJke
4. Summary Measurements of the KLL carbon Auger peak shape give valuable information on the chemical environment of carbon on the surface of a (001) diamond crystal. The typical diamond Auger peak shape changes to the graphite form under 2 keV Arf ion sputtering in the order of one minute. The percentage contribution of diamond to the measured peak decreases exponentially with sputtering time. This correlates well with a realistic transformation probability of sp3 to sp’ carbon bond per impinging ion. Therefore after some sputtering one will not be able to see the diamond peak shape any more. This was supported by the depth profile on the titanium/diamond interface. The carbide formation at the titanium/diamond interface upon annealing, which is regarded as a prerequisite for good ohmic contacts, was also observed and was quantitatively correlated by the relative carbide and graphite contributions to the measured carbon Auger peak shape upon sputtering through the Ti-diamond interface.
Science lOO/ 101 (19%) 612-616
References [I] LE. Davis. N.C. MacDonald,
P.W. Palmberg, G.E. Riach and R.E. Webber, Handbook of Auger Electron Spectroscopy, 2nd ed. (Perkin-Elmer, Physical Electronics Division. Eden Prairie, MN. 1976). [2] P. Koschar. H.T. Frischkorn, K.O. Groeneveld and G. Szabo, Z. Phys. A 315 (1983) 13. 131J.J. Bellina and M.V. Zeller. Appl. Surf. Sci. 25 (1986) 380. [Jl S. Ferrer, F. Comin. J.A. Martin, L. Vazquez and P. Bernard. Surf. Sci. 251/252 (1991) 960. [51 L. Ferrari. E. Cappelli, A. Cricenti. S. S&i. S. Polini and G. Chiarotti. Appl. Surf. Sci. 56-58 (1992) 100. 161P.E. Viljoen. E.S. Lambers and P.H. Holloway. J. Vat. Sci. Technol. B 12 (1994) 2997. 171 R.C. Burns. V. Cvethovic. C.N. Dodge, D.J.F. Evans. M.T. Rooney. P.M. Spear and C.M. Welbourn. J. Cryst. Growth 104 (1990) 257. [81 E. Taglauer, Ion Scattering Spectroscopy in Ion Spectroscopies for Surface Analysis. Methods of Surface Characterization Series. Vol. 2, Eds. A. Czandema and D.M. Hercules (Plenum. New York. 1991). 191C. Kittel. Introduction to Solid State Physics. 3rd ed. (Wiley. New York, 1967). [lOI J.S. Chen. E. Kolawa, M.-A. Nicolet and F.S. Pool, Thin Solid Films 237 (1993) 72.
&
applied
surface science
EL/SEWER
Applied Surface Science lOO/lOl
(1996) 612-616
Carbon Auger peak shape measurements in the characterization reactions on (001) diamond
of
P.E. Viljoen a3*, W.D. Roos a, H.C. Swart a, P.H. Holloway b a Department ofPhysics, ’ Department
of Materials
UOFS. P.O. Box 339. Bloemfontein
Science and Engineerin,g.
Received 22 August
Unirersi&
1995; accepted
of Florida,
17 November
9300. South Africa Gainewille.
FL 32611.
USA
1995
Abstract The measurements of Auger peak shape changes and peak shifts as a fingerprint for chemical changes on solid surfaces are well known. The carbon Auger peak shape changes have been measured to characterize reactions on a (001) diamond single crystal surface. Firstly the influence of Ar+ ion sputtering on the diamond surface was investigated. Upon sputtering the surface, the carbon Auger peak shape changes from the typical diamond to the graphite form within about one minute sputtering. The peak form change with time has been correlated with different diamond and graphite contributions. The decrease of the diamond contribution with sputter time has been described in terms of a transition probability of the carbon sp3 to sp’ bond. Secondly the formation of TIC at the interface of a Ti overlayer and the diamond substrate upon annealing, in order to produce good and stable ohmic contacts to the diamond, was confirmed by comparing the measured data with data simulated by calculation of the carbide and graphite contributions to the final carbon (KLL) peak shape.
1. Introduction In the last three
decades
the carbon
KLL
Auger
has been measured and extensively used to determine the chemical state of carbon on solid surfaces. The Auger Handbook [1] gives the most prominent shapes, e.g. carbide and graphite although other shapes have been reported in the literature. Of the earlier pioneer work is well cited in the literature. The influence of sputtering on the peak shape of carbon was investigated by Koscher et al. [2] and Bellina and Zeller [3]. The carbon (KLL) peak shape changes of diamond films have been studied by peak shape
Ferrer et al. [4] and Ferrari et al. [S]. Most of the work mentioned were of a qualitative nature. Only in a limited number of cases have quantitative methods been used. In this study the carbon (KLL) Auger peak shape have been measured on the (001) surface of diamond and quantitative methods have been used to describe the observed spectra in the characterization of reactions on the surface.
2. Experiment
The full experimental details were described elsewhere [6] and only a brief description will be given
’ Corresponding author. E-mail: [email protected]. 0 169.4332/96/$15.00 Copyright PII SO169-4332(96)00349-2
here. The single
0 1996 Elsevier Science B.V. All rights reserved
crystal
diamond,
equivalent
to a
P.E. Viljoen et al/Applied
Surfiice Science lOO/ 101 (1996) 612-616
class IIb natural diamond, was a boron doped p-type semiconductor. It was synthetically grown by a high high pressure technique [7]. It was temperature. kindly supplied by the De Beers of South Africa. The orientation was done with the back reflection Laue method. The two larger flat surfaces of the crystal, in the form of a matchbox, are parallel to the (001) crystallographic plane. Measurements were done on these surfaces. Both the Ti and Au layers were sputter deposited in a diffusion pumped vacuum system with a base pressure of about 8 X lop7 Torr. A USGun planar magnetron with a 5 cm diameter target at 20 mTorr Ar pressure, was used. The deposition rate was about 0.4 rim/s.. The film thicknesses were determined by a Sloan Dektak profilometer and checked by calculations from RBS measurements. Auger measurements were made in a PHI model 545 UHV system using a single pass CMA with the electron gun operated at 1 PA and 5 keV. In this system the sample was sputtered with 2 keV Ar+ ions by backfilling the system with pure Ar gas to 5 X 10e5 Torr. Auger spectra and depth profiles were also measured with a Perkin-Elmer PHI model 660 SAM using 10 keV electrons with a 50 nA beam current. In this case the sputtering was done with 3 keV Arf ions from a differentially pumped ion gun incident on the target at about 45” and also rastered over the surface. Annealing of the sample was done in a tube furnace under an atmosphere of 99.99% pure forming gas consisting of 90% N, and 10% HZ.
3. Results and discussion Fig. 1 shows the carbon KLL Auger peak for the diamond (001) surface for sputtering times from 0 to 60 s as indicated. The change from the typical diamond shape for no sputtering to the graphite shape after 60 s sputtering is evident. The Ar peak at 215 eV increases with sputtering time. Great care has been taken to ensure that the peak shape change was due to phase changes in the diamond and not to adsorbed species. The diamond peak shape was monitored and did not change even after two weeks in the UHV system during which time several Ar gas flushes and sputtering of the samples holder took
613
G?
.z 3
e J g 2
W
z 200
C(KLL)
220
Auger
peak
240
260
280
300
320
Electron energy (eV) Fig. 1. The measured carbon KLL Auger peak for the diamond (001) surface at sputtering times as indicated. The surface was sputtered with 2 keV Ar + ions rastered over the surface.
place. Under sputtering of the diamond surface the peak changes within seconds. This is in accordance with other observations. In order to quantify the results, standard spectra for diamond and graphite were obtained. The diamond peak was measured on a clean unsputtered diamond surface. Only a very small amount of oxygen was present on the surface. The graphite peak shape was obtained from the Auger Handbook [l]. The relative peak heights of the diamond and graphite were established by measuring the pure diamond peak before and after extensive sputtering to make sure that the surface was totally graphitized. The standards are shown in Fig. 2. The respective contributions of diamond and graphite to a specific peak in Fig. 1, was done by a Fortran program which makes use of the E04CCF NAG routine to minimize the function F(X) with N dependents X = (X,, X,, .. .. X,) using the Simplex method. It is an iterative process that does not need any differentials of the function. The value of the given function is determined in the fit as the sum of difference squared e.g.
(1) where M(X) is the measured spectrum and D(X) and G(X) respectively the diamond and graphite standards at the specific X or energy value. The fitting parameters N, and N2 are then determined by
614
P.E. Viljoen et al./Applied
-
Standards
-
- -
Surface Science 100 / 101 (19961612-616
Diamond Graphite
Diamond 0.8 i\
220
200
280
260
240
300
.
20
Electron energy (eV)
an iterative process. The sum of these two parameters for a binary alloy should be equal to 1.0 or the percentage contributions of diamond and graphite should add up to 100%. A fit has been made to all the spectra of Fig. 1. A couple of these are shown in Fig. 3(a) and (b) indicating the measured and fitted curves. The percentages in all cases add up to about 100%. These fits are all fairly good and the percentage diamond contribution to the measured peak, as determined from the fitted data, is shown in Fig. 4 as a function
-
---
(4
Measured Fitted
_ Sputtertime -
-
= 0 sec. 82% diamood:17% graphite
Sputter time = 20 see. dismond:66% graphite
_ 32%
I 200
220
’
I 240
’
1
’
260
40
Sputter
Fig. 2. The standard carbon KLL Auger spectra for the diamond and graphite. The diamond peak was measured on a clean unsputtered diamond surface. The graphite peak was taken from Ref. 111.
I 280
’ 300
Electron energy (ev) Fig. 3. The measured and fitted curves for the carbon KLL Auger peak for the diamond (001) surface at sputtering times as indicated in (a) and (b). The surface was sputtered with 2 keV Ar+ ions rastered over the surface. The fitted curves were calculated with F.q (1) by using the standard spectra.
(001)
’
60
80
100
120
140
time (seconds)
Fig. 4. The calculated diamond contribution to the carbon KLL Auger spectra as a function of the sputtering time. The solid line is a non-linear least squares fit of Eq. (2) to the data. The exponential decrease with sputter time is evident.
of the sputter time. Since some graphitization already exist for zero sputter time (see Fig. 3), the time in Fig. 4 has been extended back by 10.0 s to include the 100% diamond contribution point. The exponential decrease in the diamond contribution upon sputtering is quite evident as shown by the solid line. The solid line in Fig. 4 was calculated from the following equation [8]: Z=I,exp(
-air)
(2)
where I is the signal from the sp3 carbon bond or diamond Auger peak shape, cr the transformation probability of the sp3 to the sp’ carbon bond, i is the ion current density and the product i ’ r is the fluence of Ar ions onto the surface. According to Taglauer [8] Eq. (2) comes from the fact that the number of diamond (or sp3 bonding) carbon atoms transformed into graphite (sp’ bonding) at a rate that is proportional to the number (or fluence it) of ions hitting the surface, that is the sputtering time. From a fit of this equation to the data by a non-linear least squares method, as shown in Fig. 4, a value of ui is determined as 0.027 18 s-‘. The ion current density i was 50 X 10-l A m-’ or 3.125 X 10” ions sP ’ rn-‘. One can express the ion current density in terms of the number of ions impinging per surface atom. The density of surface atoms on the (001) diamond surface is 1.58 X lOI m-’ [9]. Thus i = 0.2 ions per surface atom per second. This gives
P.E. Vi&en et al./Applied
Stfuce
a value for u of 0.14 transformations per incoming ion. This is an estimated value which is of the right order of magnitude of what one would expect for an ion beam of this nature. Ohmic contact to the (001) surface of this diamond was established by sputter deposition of a 200 nm titanium layer followed by a 100 nm gold layer on top of the Ti to prevent oxidation during the annealing and handling process. The sample was then annealed for different times at different temperatures [6]. Very good ohmic contact was achieved by annealing the sample at 750°C for about 10 min or more in an inert atmosphere of 99.99% pure forming gas. A depth profile, Fig. 5, of the sample after annealing at 750°C for 60 min, shows amongst other things that, at the diamond/titanium interface, the KLL carbon Auger peak shape changes from a carbide rich to a graphite form as one sputters through the interface. This is clearly shown by Fig. 6 which indicate complete Auger spectra of the sample at sputtering times, as indicated, see also Fig. 5. The fact that the depth profile does not indicate a sharp interface is due to the unevenness of the surface as explained elsewhere [6]. The inserts in Fig. 6 next to the carbon peaks in (al and (b), are carbon peaks simulated by calculating linear combinations of carbide and graphite con-
0
0
20
40
60
80
100
120
140
Sputter time (min) Fig. 5. Depth profile for a Au on Ti on diamond (001) sample after annealing at 750°C for 60 min. In this case the Ti and Au layers were respectively 200 and 250 nm thick. Ti(1) and Tic?) are respectively the Tic387 eV) and the Tic418 eV) Auger peaks. Sputtering was done by a differentially pumped ion gun with 3 keV Ar+ ions rastered over the surface.
Science lOO/lOl(19951612-616
615
(h)
(
1 (c)
0
. 0
, . , . , . , . , . , . 100 200 300 400 500 600 700
Electron energy (eV) Fig. 6. Auger spectra from Au on Ti on diamond (001) sample annealed at 750°C for 60 min after sputtering times of (a) 100. (b) 120, and (c) 140 min (see Fig. 5). The insets in (a) and (b) are the simulated C peaks for carbide contributions to the peak of 50% and 10%. respectively. The carbon peak in (c) is from graphite, which results from the continuous ion bombardment of diamond surfaces.
tributions to the final peak shape. The carbide peak shape which has been used as standard, was taken from the Auger Handbook [l] for TaC which is a very typical carbide shape. The relative peak heights were determined from the peak heights and amplification factors in the PHI Auger Handbook [l]. The calculated and measured peak shapes are very similar and the percentage contribution can be considered as fairly accurate. It clearly shows the existence of carbide at the interface which, upon sputtering into the substrate, disappear and only the sputter converted graphite form of the diamond peak remains. Calculation with Eq. (1) could be performed in this case, however, it becomes much more difficult due to the larger number of elements present. Since the result in this case is more qualitative in nature, we only used the linear combinations. The existence of carbide at the interface indicates the formation of TIC which is a prerequisite for good ohmic contacts. This was always suggested but the proof for that is known only in a few cases. This is also in agreement with RBS measurements [6,10].
616
P.E. Viljoert et &./Applied
SurJke
4. Summary Measurements of the KLL carbon Auger peak shape give valuable information on the chemical environment of carbon on the surface of a (001) diamond crystal. The typical diamond Auger peak shape changes to the graphite form under 2 keV Arf ion sputtering in the order of one minute. The percentage contribution of diamond to the measured peak decreases exponentially with sputtering time. This correlates well with a realistic transformation probability of sp3 to sp’ carbon bond per impinging ion. Therefore after some sputtering one will not be able to see the diamond peak shape any more. This was supported by the depth profile on the titanium/diamond interface. The carbide formation at the titanium/diamond interface upon annealing, which is regarded as a prerequisite for good ohmic contacts, was also observed and was quantitatively correlated by the relative carbide and graphite contributions to the measured carbon Auger peak shape upon sputtering through the Ti-diamond interface.
Science lOO/ 101 (19%) 612-616
References [I] LE. Davis. N.C. MacDonald,
P.W. Palmberg, G.E. Riach and R.E. Webber, Handbook of Auger Electron Spectroscopy, 2nd ed. (Perkin-Elmer, Physical Electronics Division. Eden Prairie, MN. 1976). [2] P. Koschar. H.T. Frischkorn, K.O. Groeneveld and G. Szabo, Z. Phys. A 315 (1983) 13. 131J.J. Bellina and M.V. Zeller. Appl. Surf. Sci. 25 (1986) 380. [Jl S. Ferrer, F. Comin. J.A. Martin, L. Vazquez and P. Bernard. Surf. Sci. 251/252 (1991) 960. [51 L. Ferrari. E. Cappelli, A. Cricenti. S. S&i. S. Polini and G. Chiarotti. Appl. Surf. Sci. 56-58 (1992) 100. 161P.E. Viljoen. E.S. Lambers and P.H. Holloway. J. Vat. Sci. Technol. B 12 (1994) 2997. 171 R.C. Burns. V. Cvethovic. C.N. Dodge, D.J.F. Evans. M.T. Rooney. P.M. Spear and C.M. Welbourn. J. Cryst. Growth 104 (1990) 257. [81 E. Taglauer, Ion Scattering Spectroscopy in Ion Spectroscopies for Surface Analysis. Methods of Surface Characterization Series. Vol. 2, Eds. A. Czandema and D.M. Hercules (Plenum. New York. 1991). 191C. Kittel. Introduction to Solid State Physics. 3rd ed. (Wiley. New York, 1967). [lOI J.S. Chen. E. Kolawa, M.-A. Nicolet and F.S. Pool, Thin Solid Films 237 (1993) 72.