- Email: [email protected]
Analysis of Auger sputter depth profiles with a resolution function
._.....
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“i:.:,~,~l,~r
:,:,(
F
.....,.i
~~
_,.__
I....
.,.,:,
surfacescience ELSEWIER
Applied Surface Science lOO/lOl (1996) 89-91
Analysis of Auger sputter depth profiles with a resolution function T. Kitada
*, T.
Harada, S. Tanuma
Japan Energy ARC. Co. Ltd., 3-l 7-35 Niko-Minami,
Toda, Saitama 335, Japan
Received 18 August 1995
Abstract We have determined the depth resolution function from the Auger sputter depth profiles of GaAs/AlGaAs specimens which have abrupt interfaces. We have also applied the obtained resolution function to the analysis of a GaAs/AlGaAs sample containing aluminium graded-layers in order to know the effectiveness of the depth resolution function. The resulting aluminum graded-layer thickness is about 14 nm, which is in good agreement with the values estimated from the growth rate of the thin layer at preparing the specimen with molecular beam epitaxy. The resulting resolution function can be fitted by three parameters: atomic mixing, surface roughness and information depth, which was proposed by Hofmann.
1. Introduction Auger
analysis
combined
with Argon
ion sputter-
ing is widely used for the evaluation of semiconductor multilayer structures. In practical cases, we find broader interfaces in the Auger sputter depth profiles than the ‘true’ ones due to atomic mixing, surface roughness which was caused by the ion sputtering, and effects from the inelastic mean free paths. It is, therefore, difficult to evaluate the structure of a semiconductor multilayer including thin gradedlayers by AES. The observed Auger depth profile can be expressed as [l]
where Y(t) is the observed
depth profile,
h(t) is the
* Corresponding author. Tel.: + 81-48-4332144; fax: + 81-484421845: e-mail: [email protected].
depth resolution function, and x(t) is the true concentration depth profile of the sample. From Eq. (I), we can get a depth resolution function (DRF) if the true concentration profile x is a step function, I$) = h(,, c3
$y =h(,) c3 6 = h(,,.
(2)
We have determined a depth resolution function using Eq. (2) and applied it to the analysis of the structure of semiconductor multilayers, which include Al thin graded-layer samples, by AES.
2. Experimental Auger depth profiling analyses of the specimens were carried out with 1 kV Ar+ ion sputtering. The incident angle of the ion beam was 35 degrees from the sample surfaces. The measurements were made with the JEOL JAMP-30 scanning Auger microprobe. The accelerating voltage primary electron
0169-4332/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. PII SO169-4332(96)00359-5
T. Kitada et al. /Applied
90
Surface Science lOO/ IO!
f 1996189-91 Resolution
beam was 5 kV, its current 0.1 ~.LAand the measured Auger energy range was 25-85 eV (0.5 eV steps). The specimens used for the measurements were GaAs (30 nm)/Al,,,,Ga,,,, As (640 nm)/GaAs substrate (the reference sample) and a graded-layer sample which was grown on the GaAs substrates by molecular beam epitaxy (MBE) (GaAs (30 nm)/Al.,GaC, nm)/GaAs). 10 nm.
_,,As (10 nm)/A1,,,2Ga0.78 As (640 The range of x is O-O.22 between ca.
3. Results and discussion 3.1. Applicatiotl
to analysis of the graded-layer
sam-
ple The measured Auger depth profile of the reference sample was shown in Fig. 1A. We could point out that the observed Auger intensity of Al contained the effect of the Ga Auger peak from the GaAs region. We carried out, then, the peak separation of the overlapped peak to the components of GaAs and AlGaAs with the non-negative least square curve fitting method. The result is shown in Fig. 1B. Using the logistic function [2], we have removed the noise from the Auger depth profile of the GaAs component. We have also obtained the depth resolution function using Eq. (2) as shown in Fig. 2. We calculated the Auger depth profile of the GaAs component of the graded-layer with the convolution method using the resolution function. As the model function for the concentration we prepared 10 nm and 14 nm layer structures, which are sketched
Fig. 2. The depth resolution Eq. (2).
function
~~~~~=
3.2. Calculation
We could not describe the resolution function with a simple function such as a Gaussian. However, the left side of the resolution function could be fitted with a Gaussian function and the right one could be fitted with a Lorentzian one, respectively, as shown in Fig. 4A. Hofmann pointed out that the resolution function of the Auger depth profile can be expressed as three parameters relating to atomic mixing, surface roughness and information depth [3]. The effect of surface
a 1.0
1.0
z
0.8
0.8
2 0.6
0.6
2
0.4 0.2
$
20 Depth
46 (nm)
60
fz
0
20 Depth
40
0.4
(nm)
0.0 0
20
40
Depth(nm)
60
Fig. 1. The Auger depth profiles of a GaAs/AlGaAs sample (reference sample). (A) is the raw depth profile and (B) is the calculated results with the non-negative least square peak separation method.
from Fig. LB by
of the resolution function
& 0.2 2 0.0
0
obtained
out in Fig. 3A. The resulting profile is shown in Fig. 3B. In the figures, the solid line expresses the result of a 14 nm layer thickness structure and the dotted line that of a 10 nm one. The open circles represent the observed Auger depth profiles. The convolution result of the 14 nm layer thickness structure coincides well with the observed data. We could conclude that the thickness of the Al graded layer of the specimen is 14 nm.
z in
function
60
20 40 Depth(nm)
60
Fig. 3. The structures of the Al graded layers. (A) is the gradedlayer model function. (B) is the calculated resolution function using the model function (A) in the convolution method (solid and dotted lines). The open circles represent the observed Auger depth profile.
T. Kitada et nl./Applied
Surface Science 100/101(1996)
information
89-91
91
depth:
ID = exp - f , ( 1
Depth (nm)
Depth (nm)
Fig. 4. Calculated resolution functions, (A) shows the curve fit results with two functions: Gaussian (left side) and Lorentzian (right side). (BJ is the results of Hofmann’s method (1.8 nm for vv, 1.5 nm for o and 0.5 nm for A). Open circles represent the depth resolution function obtained from the observed Auger depth profile by Eiq. (2). The solid lines correspond to the calculated depth resolution functions.
roughness could be expressed as a Gaussian function and the other two effect are described as exponential decay form functions. They are as follows: atomic mixing:
where w is the atomic mixing depth (nm), u is the surface roughness (nm>, X is the inelastic mean free path (nm) and z is the depth from the surface (nm>. We have obtained a resolution function using Hofmann’s method mentioned above. The resulting function is shown in Fig. 4B. In this figure, the solid line is the resolution function and the open circles are the result of convolution of Hofmann’s method. When the parameters for Eqs. (3)-(5) are 1.8 nm for w, 1.5 nm for u and 0.5 nm for A, the calculated resolution function with Hofmann’s method is in good agreement with the one obtained by Eq. (2) as shown in Fig. 4B.
References AM=exp(-y),
(3)
surface roughness: .SR=exp(
-ln2(
t,‘),
[l] P.A. Jansson, Deconvolution with Applications troscopy (Academic Press, New York, 1984). [2] W.H. Kirchhoff, Logistic function data analysis MSTJR 88-3803 (1989). [3] S. Hofmann, Surf. Interf. Anal. 673 (1994) 21.
in Specprogram.
..I.~...~~~.....~
,.,.,..
‘.‘.‘.‘.:d....n,m . .......:.:.:.:.,.,.,...........:,:q> :.=...:.:.:.x v. ..:.:.:.> ..,.... :,:,> ,...,.....,........ g~~~~.~:“.: ...A.I’.‘...... ,(..... ..‘.‘..“...........:,:.:~:~,::r:
“i:.:,~,~l,~r
:,:,(
F
.....,.i
~~
_,.__
I....
.,.,:,
surfacescience ELSEWIER
Applied Surface Science lOO/lOl (1996) 89-91
Analysis of Auger sputter depth profiles with a resolution function T. Kitada
*, T.
Harada, S. Tanuma
Japan Energy ARC. Co. Ltd., 3-l 7-35 Niko-Minami,
Toda, Saitama 335, Japan
Received 18 August 1995
Abstract We have determined the depth resolution function from the Auger sputter depth profiles of GaAs/AlGaAs specimens which have abrupt interfaces. We have also applied the obtained resolution function to the analysis of a GaAs/AlGaAs sample containing aluminium graded-layers in order to know the effectiveness of the depth resolution function. The resulting aluminum graded-layer thickness is about 14 nm, which is in good agreement with the values estimated from the growth rate of the thin layer at preparing the specimen with molecular beam epitaxy. The resulting resolution function can be fitted by three parameters: atomic mixing, surface roughness and information depth, which was proposed by Hofmann.
1. Introduction Auger
analysis
combined
with Argon
ion sputter-
ing is widely used for the evaluation of semiconductor multilayer structures. In practical cases, we find broader interfaces in the Auger sputter depth profiles than the ‘true’ ones due to atomic mixing, surface roughness which was caused by the ion sputtering, and effects from the inelastic mean free paths. It is, therefore, difficult to evaluate the structure of a semiconductor multilayer including thin gradedlayers by AES. The observed Auger depth profile can be expressed as [l]
where Y(t) is the observed
depth profile,
h(t) is the
* Corresponding author. Tel.: + 81-48-4332144; fax: + 81-484421845: e-mail: [email protected].
depth resolution function, and x(t) is the true concentration depth profile of the sample. From Eq. (I), we can get a depth resolution function (DRF) if the true concentration profile x is a step function, I$) = h(,, c3
$y =h(,) c3 6 = h(,,.
(2)
We have determined a depth resolution function using Eq. (2) and applied it to the analysis of the structure of semiconductor multilayers, which include Al thin graded-layer samples, by AES.
2. Experimental Auger depth profiling analyses of the specimens were carried out with 1 kV Ar+ ion sputtering. The incident angle of the ion beam was 35 degrees from the sample surfaces. The measurements were made with the JEOL JAMP-30 scanning Auger microprobe. The accelerating voltage primary electron
0169-4332/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. PII SO169-4332(96)00359-5
T. Kitada et al. /Applied
90
Surface Science lOO/ IO!
f 1996189-91 Resolution
beam was 5 kV, its current 0.1 ~.LAand the measured Auger energy range was 25-85 eV (0.5 eV steps). The specimens used for the measurements were GaAs (30 nm)/Al,,,,Ga,,,, As (640 nm)/GaAs substrate (the reference sample) and a graded-layer sample which was grown on the GaAs substrates by molecular beam epitaxy (MBE) (GaAs (30 nm)/Al.,GaC, nm)/GaAs). 10 nm.
_,,As (10 nm)/A1,,,2Ga0.78 As (640 The range of x is O-O.22 between ca.
3. Results and discussion 3.1. Applicatiotl
to analysis of the graded-layer
sam-
ple The measured Auger depth profile of the reference sample was shown in Fig. 1A. We could point out that the observed Auger intensity of Al contained the effect of the Ga Auger peak from the GaAs region. We carried out, then, the peak separation of the overlapped peak to the components of GaAs and AlGaAs with the non-negative least square curve fitting method. The result is shown in Fig. 1B. Using the logistic function [2], we have removed the noise from the Auger depth profile of the GaAs component. We have also obtained the depth resolution function using Eq. (2) as shown in Fig. 2. We calculated the Auger depth profile of the GaAs component of the graded-layer with the convolution method using the resolution function. As the model function for the concentration we prepared 10 nm and 14 nm layer structures, which are sketched
Fig. 2. The depth resolution Eq. (2).
function
~~~~~=
3.2. Calculation
We could not describe the resolution function with a simple function such as a Gaussian. However, the left side of the resolution function could be fitted with a Gaussian function and the right one could be fitted with a Lorentzian one, respectively, as shown in Fig. 4A. Hofmann pointed out that the resolution function of the Auger depth profile can be expressed as three parameters relating to atomic mixing, surface roughness and information depth [3]. The effect of surface
a 1.0
1.0
z
0.8
0.8
2 0.6
0.6
2
0.4 0.2
$
20 Depth
46 (nm)
60
fz
0
20 Depth
40
0.4
(nm)
0.0 0
20
40
Depth(nm)
60
Fig. 1. The Auger depth profiles of a GaAs/AlGaAs sample (reference sample). (A) is the raw depth profile and (B) is the calculated results with the non-negative least square peak separation method.
from Fig. LB by
of the resolution function
& 0.2 2 0.0
0
obtained
out in Fig. 3A. The resulting profile is shown in Fig. 3B. In the figures, the solid line expresses the result of a 14 nm layer thickness structure and the dotted line that of a 10 nm one. The open circles represent the observed Auger depth profiles. The convolution result of the 14 nm layer thickness structure coincides well with the observed data. We could conclude that the thickness of the Al graded layer of the specimen is 14 nm.
z in
function
60
20 40 Depth(nm)
60
Fig. 3. The structures of the Al graded layers. (A) is the gradedlayer model function. (B) is the calculated resolution function using the model function (A) in the convolution method (solid and dotted lines). The open circles represent the observed Auger depth profile.
T. Kitada et nl./Applied
Surface Science 100/101(1996)
information
89-91
91
depth:
ID = exp - f , ( 1
Depth (nm)
Depth (nm)
Fig. 4. Calculated resolution functions, (A) shows the curve fit results with two functions: Gaussian (left side) and Lorentzian (right side). (BJ is the results of Hofmann’s method (1.8 nm for vv, 1.5 nm for o and 0.5 nm for A). Open circles represent the depth resolution function obtained from the observed Auger depth profile by Eiq. (2). The solid lines correspond to the calculated depth resolution functions.
roughness could be expressed as a Gaussian function and the other two effect are described as exponential decay form functions. They are as follows: atomic mixing:
where w is the atomic mixing depth (nm), u is the surface roughness (nm>, X is the inelastic mean free path (nm) and z is the depth from the surface (nm>. We have obtained a resolution function using Hofmann’s method mentioned above. The resulting function is shown in Fig. 4B. In this figure, the solid line is the resolution function and the open circles are the result of convolution of Hofmann’s method. When the parameters for Eqs. (3)-(5) are 1.8 nm for w, 1.5 nm for u and 0.5 nm for A, the calculated resolution function with Hofmann’s method is in good agreement with the one obtained by Eq. (2) as shown in Fig. 4B.
References AM=exp(-y),
(3)
surface roughness: .SR=exp(
-ln2(
t,‘),
[l] P.A. Jansson, Deconvolution with Applications troscopy (Academic Press, New York, 1984). [2] W.H. Kirchhoff, Logistic function data analysis MSTJR 88-3803 (1989). [3] S. Hofmann, Surf. Interf. Anal. 673 (1994) 21.
in Specprogram.