SURFACE AND INTERFACE ANALYSIS, VOL. 22, 22-26 (1994)

Crystalline Effects on Depth Resolution in AES Depth Profiling

~-

K. Kajiwara Sony Corp. Yokohama TEC Research Center, 134 Goudo-cho, Hodogaya-ku, Yokohama-shi 240 Japan

An evaluation of the depth resolution of sputter-assisted Auger electron spectroscopy (AES) has been studied for semiconductor superlattices. Zalar et al. have shown that sample rotation is effective in improving the depth resolution of AES depth profiling. Multilayers with two kinds of crystalline states were prepared to test the effectiveness of sample rotation. First, a microcrystalline Zr/Nb (10 nm/lO nm) multilayer was deposited by dc-sputtering. Secondly, an AIAs/GaAs (10 nm/9 nm) and an InP/InGaAs (3 nm/3 nm) single crystalline superlattice were grown by metal-organic chemical vapor deposition.

AES depth profiling with Ar' ion sputtering was performed on these multilayers with and without sample rotation. Comparing the rotational depth profiling results with those obtained on stationary samples, it is found that, in the cases of the Zr/Nb and the InP/InGaAs multilayers, the depth resolution was considerably improved by sample rotation while in the case of AlAs/GaAs multilayer the depth resolution was only little improved by sample rotation. It is shown that the effectiveness of sample rotation strongly depends on the crystalline states of samples and on the degree of preferential sputtering.

INTRODUCTION

An evaluation of the depth resolution of sputter-assisted Auger electron spectroscopy (AES) has been studied for semiconductor superlattices.'X2 Zalar et al. have shown that sample rotation is effective in improving the depth resolution of AES depth profiling of Ni/Cr (e.g. 30 nm/ 30 nm) m~ltilayers.~-' In this case, metallic multilayers are deposited in a polycrystalline state and variously oriented grains are included in the probing area (e.g. 50pm $) of AES. Sample rotation equilibrates the different sputtering rates of the variously oriented grains during ion sputtering. This subsequently reduces sputter- induced roughness and, therefore enhances the depth resolution.

In recent years Zr/Nb multilayers have been studied as superconducting metals which have novel proper tie^.'.^ In order to confirm quantitatively the improvement of depth resolution by using sample rotation, a micro- crystalline Zr/Nb multilayer film was deposited by magnetron dc-sputtering on a Si substrate. It is supposed that the sputter-induced roughness of the Zr/Nb multi- layer is considerably reduced by sample rotation.

Because semiconductor superlattices, e.g. AlAs/GaAs superlattices, are grown in a single crystalline state, sputtering rate in them has a single value when sputtering conditions are constant. Therefore it is supposed that the sputter-induced roughness of the AlAs/GaAs superlattice is little reduced by sample rotation.

Whereas in the AlAs/GaAs systems Ar' ion bombard- ment gives rise to a small preferential sputtering of arsenic and the sputtered surface is not roughened,'.'' InP/ InGaAs systems among semiconductor superlattices are well known to be roughened by Ar+ ion bombardment due to the large preferential sputtering of phosphorus

CCC 0142 -2421/94/240022 -05 T, 1994 by John Wiley & Son\, Ltd

followed by island formation of the excess indium.' ',12 It is suggested, therefore, that the sputter-induced roughness of InP/InGaAs superlattices is considerably reduced by sample rotation in spite of their single crystalline state.

In this article, these crystalline effects on depth resolution are investigated with AES depth profiling with and without sample rotation.

EXPERIMENTAL

Sample preparation

Multilayers with two kinds of crystalline states were prepared in order to test the effectiveness of sample rotation. A Zr/Nb (10 nm/lO nm, six periods) multilayer with a total thickness of 120 nm and with a micro- crystalline structure was deposited by magnetron dc- sputtering on a Si (1 11) single crystal substrate. It was confirmed by small angle x-ray diffractometry that the Zr/Nb multilayer has an atomically abrupt and flat interface, and that the grain size is approximately 10 nm.

An AlAs/GaAs (10 nm/9 nm, five periods) superlattice with single crystalline state was epitaxially grown by metal-organic chemical vapor deposition (MOCVD) on a GaAs (100) single crystal ~ubs t ra te . '~ Figure 1 shows a lattice image of the AlAs/GaAs superlattice obtained by cross-sectional transmission electron microscopy (TEM). The AlAs/GaAs superlattice has an atomically 'abrupt interface and an atomically flat surface on the otder of one atomic layer. In addition, an InP/In,Ga, -,As superlattice ( 3 nm/3 nm, x = 0.53, 170 periods, total thickness - 1 pm) with single crystalline state was also grown by MOCVD on an InP (100) substrate.

CRYSTALLINE EFFECTS O N DEPTH RESOLUTION IN AES DEPTH PROFILING 23

Figure 1. Lattice image of an AIAs(l0 nm)/GaAs(S nm) super- lattice obtained by transmission electron microscopy.

Depth profiling analysis by AES

AES depth profiling with Ar' ion sputtering was performed on these multilayers by the stationary method and by sample rotation in a JEOL JAMP-1OS scanning Auger microprobe system. Sample rotation was operated at 1/6 rpmI4 and probing areas coincided with the rotation center as exactly as possible, e.g. within the range of k 5 pm.

Ar' ion etching was performed by using a differentially pumped type ion gun incident at 55" to the sample surface normal. Ion etching conditions were as follows: ion energy 1 keV; Ar gas pressure 2 x 10V2 Pa; ion current - 1 x A; raster scanned area 0.7 x 1 mm2; etching rate 0.4-0.5 nm min-'.

AES measurement conditions were: electron acceler- ating voltage 2 kV; probe current -2 x A; probe diameter 50 pm; modulation voltage 3 eV,,. Auger signals used here were Zr MNN (147 eV), Nb MNN (1 67 eV) and 0 KLL (510 eV) for Nb/Zr depth profiling; As MNN (33 eV), Ga MMM (55 eV) and A1 LVV (65 eV) for AlAs/GaAs and As MNN (33 eV), Ga MMM (55 eV), P LVV (120 eV) and In MNN (404 eV) for InP/InGaAs.

In this work, the definition of depth resolution follows ASTM E673-90, i.e. the distance over which a 16 to 84% (or 84 to 16%) change in signal is measured."

RESULTS AND DISCUSSION

Zr/Nb multilayer

A measurement of a Zr( 10 nm)/Nb( 10 nm) multilayer made with stationary method is shown in Fig. 2, and that of the same sample with sample rotation is shown in Fig. 3. In both figures, it is noted that the Zr MNN Auger intensities increase in each Nb layer since the Nb MNN sub-peak (142eV) overlaps with the Zr MNN peak

> In f aJ C

..- .-

+- - L a, cn Y 6

0 50 100 Depth (nm)

Figure 2. Auger depth profile of a Zr ( l0 nm)/Nb(lO rim) layer obtained on stationary sample.

multi-

Nb >.

In C Q,

f

c .-

c I

L- aJ 01 Y 6

0 50 100 Depth ( n m )

Figure 3. Auger depth profile on a Zr( l0 nm)/Nb(lO nm) multi- layer obtained with sample rotation.

region (142-150 eV). The true Zr MNN intensity can be distinguished from the overlapping spectrum by curve fitting, e.g. factor analysis method.I6

Figures 2 and 3 show that the depth resolution Az is considerably improved by sample rotation of the Zr/Nb multilayer as Zalar et al. have shown on Cr/Ni multi- layer^.^-^ It is also clear that the depth resolution of the Zr(upper)/Nb(lower) side is narrower than that of Nb/Zr side, e.g. Az = 2.6 nm and 3.7 nm respectively. This suggests that Zr atoms have a higher sputtering yield than Nb atoms under Ar + ion bombardment since atomic mixing or interdiffusion due to ion bombardment have almost the same effect on both interfaces.

24 K. KAJIWARA

C 0 ._ + d 3

$ 5 -

5 Q aJ 0

L

i y 10 C

- -Ac- stationary _ , - P I - - -a-

A- -o--- \ Z r / N b

,jHr AIAs/GaAs\(stat.&rot.) rotation 0' - - 0

' Z r / N b '

,N b / Zr,

0 ' " " " ' " " ' ~ 0 50 100

Sputtered depth ( n m 1 Figure 4. The dependence of depth resolution on the sputtered depth in Zr,"b and AIAs/GaAs multilayer samples.

Hofmann has shown that the total depth resolution Az can be described by the following expre~sion:"-'~

Az' = A z ~ + A z ~ + Az,Z + A z ~ + Az; + Az? + ... (1) where Azo is the original interface width, Azs surface roughening by sputtering statistics, Azk atomic mixing, Azi electron inelastic mean free path (IMFP), Az, sputter-induced roughness, Azi inhomogeneous ion beam intensity and so on. It is estimated that Az, and Azk are constant with sputtered depth while in general Az, and Azi increase with depth.

Figure 2 shows that the depth resolution of stationary samples deteriorates as the depth increases and that the total depth resolution Az can be described by the following expression : ' O p 2

AZ = CI + PZ" (2) where n ranges from lj2 to 1.

Figure 4 shows the depth resolution Az as a function of sputtered depth z . For the Zr/Nb interfaces in the stationary case,

Az nm = 2.60 + 0.032 (3)

For the Nb/Zr interfaces of the stationary sample,

Az nm = 3.70 + 0 . 0 8 ~ ~ ' ~

and for the rotated sample, Az = 2.60 nm.

(4) and for the rotated sample, Az = 3.70 nm.

The first component is constant with depth and represents broadening due to fundamental processes such as Azo, Az,, Azk and A Z ~ . The second component is a function of depth and includes Azr and Azi. Therefore, it is clear that the second component is completely eliminated by using sample rotation of the Zr/Nb multilayer.

A1AsIGaAs superlattice

A measurement of a stationary AlAs(l0 nm)/GaAs(9 nm) superlattice is shown in Fig. 5, and that of the rotated sample is shown in Fig. 6. In both figures, it is noted that

2.

ul C aJ

c .-

c C - L aJ 0 3 a

0 5 0 100 Depth ( n m )

Figure 5. Auger depth profile of an AIAs(l0 nm),;GaAs(S nm) superlattice obtained on a stationary sample.

C aJ C c - L aJ cn 3 a

0 50 100 Depth ( n m )

Figure 6. Auger depth profile of an AIAs(l0 nm)/GaAs(S nm) superlattice obtained with sample rotation.

the Ga MMM Auger intensities increase in the AlAs layers, since the A1 LVV main peak's tail overlaps with the Ga MMM peak region (48-56eV). The true Ga MMM intensity can also be distinguished from the overlapping spectrum by using the factor analysis method.I3

Figures 5 and 6 show that the depth resolution of the AlAs/GaAs superlattice is little improved by sample rotation. There is no difference between the depth resolution of the AlAs/GaAs side and that of the GaAs/AlAs side. In addition, there is no difference between the depth resolution by the stationary method

CRYSTALLINE EFFECTS ON DEPTH RESOLUTION IN AES DEPTH PROFILING 25

and that by sample rotation. This is also shown in Fig. 4 for easy comparison with the Zr/Nb multilayer. It is confirmed that, if materials are in a single crystalline state, sputter-induced roughness does not increase with the sputtered depth even on the stationary sample.

Figure 4 shows that the depth resolution is constant, having no relation to the depth z from the topmost surface, and that the total depth resolution Az can be described by the following expression for this AlAs/GaAs superlattice:

Az = 3.70 nm ( 5 ) corresponding to the first component of Eqn (2).

However, in the case of sample rotation, Auger signal intensities of A1 LVV, Ga MMM and As MNN fluctuate periodically with rotation and the A1 LVV change rate amounts to 25% of the total A1 LVV intensity in the AlAs layers. Many reasons are considered for this periodic fluctuation such as change of analysis a ~ e a , ~ ” ~ diffraction effect of Auger electron^,^ 5 , 2 6 charge imbalance due to rotational asymmetry of sample holder and so on.

Auger signal intensities of Zr MNN and Nb MNN scarcely fluctuate with sample rotation on the Zr/Nb multilayer (Fig. 3). It was confirmed that, on GaAs (100) and Si (1 11) substrates, Ga MMM, As MNN and Si LVV intensities also scarcely fluctuate with sample rotation. It is characteristic of the AlAs/GaAs systems in which AlAs layers (Se doped) have a higher resistivity than GaAs layers (Se doped, 2 x 10” cm-3) and the GaAs substrate used here (Si doped, 2 x lo’* ~ m - ~ ) . Therefore, it is concluded that the Auger intensity fluctuation on the AlAs/GaAs superlattice is caused by the charge imbalance.

In order to reduce the charge imbalance (so-called charge-up), a Au thin film (e.g. 50 nm) was deposited on the surface and the sample stage was tilted (e.g. 25 deg. from horizontal). Figure 7 shows A1 LVV depth profiles

> In C 0,

C

c .-

* I

L Q 0 3 a

0 20 40 Depth ( n m )

Figure 8. Auger depth profile of an lnP(3 nm)/lnGaAs(3 nm) sumrlattice obtained on a stationary sample.

>. VI C Q

C

+-I .-

* - L Q 0 3 a

0 20 40 Depth ( n m )

Figure 9. Auger depth profile of an lnP(3 nm)/lnGaAs(3 nm) superlattice obtained with sample rotation.

of the AlAs/GaAs superlattice with normal sample rotation and with Au film and tilt. The A1 LVV intensity fluctuation is drastically reduced from 25 to 7%.

0 10 20 30 40 50 Sampling number

Figure 7. Al LVV depth profiles of an AIAs(l0 nm)/GaAs(S nm) superlattice obtained with sample rotation; (a) normal condition and (b) with Au film (50 nm) deposited on the surface and tilt (25” from horizontal).

InP/InGaAs superlattice

A stationary measurement of an InP(3 nm)/InGaAs(3 nm) superlattice is shown in Fig. 8, and that of the rotated sample is shown in Fig. 9.

Figures 8 and 9 show that the depth resolution is considerably improved by sample rotation of the InP/

26 K. KAJIWARA

Table 1. Depth resolution of AES depth profiles obtained with- out and with sample rotation

Sample

Zr/Nb Zr/Nb N b/Zr N b/Zr Al As/GaAs Al As/Ga As InP/lnGaAs In P/lnGaAs

Crystalline state

microcrystal. microcrystal. microcrystal. microcrystal. single crystal. single crystal. single crystal. single crystal.

Rotation speed

stationary 1 /6 rpm stationary 116 rpm stationary 116 rpm stationary 116 rpm

Depth resolution

2.6 + 0.031 nm 2.6 nm, const. 3.7 + 0 . 8 ~ ~ ' ~ nm 3.7 nm, const. 3.7 nm, const. 3.7 nm, const. -3 + b f nm -3 nm, const.

InGaAs superlattice. There are two reasons why the depth resolution is improved by sample rotation. First, the InP/InGaAs superlattice has a little original surface roughness due to a slight difference of the lattice constant between InP and InGaAs. Secondly, a number of islands (perhaps excess In) are formed on the sputtered surface during measurement owing to preferential sputtering of the stationary sample.1'~'2

The depth resolution cannot be definitely determined for these cases in which a O-lOO% signal change is not obtained. The equations proposed by S. HofmannZ7 can be used to determine the depth resolution from the sputtering profiles in these cases. Figure 8 shows that on the stationary sample the depth resolution deteriorates as the depth increases. The total depth resolution Az can be described by Eqn (2), though the parameters a, /3 and n are not yet determined. It is estimated that the first component c( is approximately the same value as each layer thickness, i.e. - 3 nm.

It is noted that, on the rotated sample, In MNN and P LVV Auger intensities show irregular changes from the topmost surface to the fourth period of the superlattice. This irregularity is estimated to originate from the analysis area change and/or slight charge imbalance. In measuring such a very short periodic superlattice by using sample rotation, the Auger signal changes are composed of true compositional changes, analysis area changes and intensity fluctuations due to asymmetry. In any case, depth profiling with sample rotation requires careful alignment between the analysis area and the rotation center and requires very accurate charge neutralization.

Table 1 shows a compilation of depth resolutions in this work.

CONCLUSIONS

The AES depth profiling results obtained on stationary and rotated samples are summarized as follows:

1. In the case of the Zr/Nb multilayer with micro- crystalline state, the depth resolution is greatly improved by sample rotation as Zalar has shown on Ni/Cr multilayers.

2. As long as the sputtering conditions are optimized, in the case of the AlAs/GaAs superlattice with single crystalline state, the depth resolution is little improved by sample rotation.

3. In the case of the InP/InGaAs superlattice with single crystalline state, in which the preferential sputtering and subsequent island formation occur, the depth resolution is considerably improved by sample rotation.

The effectiveness of sample rotation strongly depends on the crystalline state of samples and on the degree of preferential sputtering. Further work, especially on multilayers with amorphous states, is necessary in order to clarify the mechanism of sputter-induced roughness and crystalline effects on the depth resolution of AES depth profiles.

Acknowledgements

The author would like to thank Professor R. Shimizu of Osaka University for valuable advice. The author thanks Dr H. Kawai and F. Nakamura for preparing AlAs/GaAs and InP/InGaAs superlattices by MOCVD, and thanks Dr N. Satoh for preparing Nb/Zr multilayers by dc-sputtering and for helpful discussions.

The author thanks Dr A. Mogami, Y. Sakai, Y. Nagasawa and Dr T. Sekine (JEOL) for making the AES apparatus and for many discussions. The author also thanks T. Yoshioka and T. Watabe (JEOL) for the TEM sample preparation and observation.

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