SURFACE AND INTERFACE ANALYSIS, VOL. 15, 4 3 H 3 9 (1990)

Analysis of AlGaAslGaAs Superlattices by Means of Sputter-assisted AES, SEM and TEM2E

K. Kajiwara and H. Kawai Sony Corp. Research Centre, 174 Fujitsuka-cho, Hodogaya-ku, Yokohama City, 240 Japan

This paper reports the developments of evaluation methods on epitaxially grown superlattices by means of sputter- assisted Auger electron spectroscopy (AES), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). AlGaAs/GaAs semiconductor superlattices were grown epitaxially by metal-organic chemical vapour deposition (MOCVD). The layer thickness of the superlattices ranged from a few to ten nanometres.

Firstly, developments of Auger depth profiling were tried by using: (1) a differential pumping-type ion gun instead of a static pressure type to reduce the oxygen adsorbates on the AlGaAs layer; (2) lowenergy Auger signals instead of highsnergy ones to shorten the escape depth; and (3) the lowest ion etching energy of 0.2 keV instead of 1 keV to reduce the surface roughening effects. It is shown that the depth resolution of sputter-assisted AES is attainable to 15 om.

Secondly, high-resolution SEM can be used as an easy evaluation method by observing the cleaved surface of superlattices, since the layers can be distinguished by signal contrast. Also, TEM can be used as an evaluation method by observing the (110) cross-section thinned sample. The dark field image has a high contrast between AlGaAs and GaAs using the (002) diffraction.

It is confirmed from these AES, SEM and TEM evaluations that the hetero-interface abruptness of AlGaAs/ GaAs superlattices grown by MOCVD is of the order of one monoatomic layer.

INTRODUCTION

In recent years, semiconductor superlattice structure devices are laminated alternately with two different ultrathin materials on single-crystal substrates have been studied actively. To realize the superlattice, an atomic-layer crystal growth technique is requisite, and at present molecular beam epitaxy (MBE)' or metal- organic chemical vapour deposition (MOCVD)' is mostly used as the fabrication method. In fact, it became possible to make superlattices in accordance with the progress of the growth technique, and applica- tions to new functional devices can be expected.

In addition, some techniques that characterize accu- rately the structure of superlattices grown by MBE or MOCVD are required to realize the superlattice. Today, a number of analytical methods have been studied to evaluate the controllability of film thickness, the abrupt- ness of chemical composition and the flatness and the crystal imperfection of the hetero-interface. Auger elec- tron spectroscopy (AES) with ion sputtering, secondary ion mass spectrometry (SIMS)3 and photolumin- escence4 are generally used to evaluate the film thick- ness and abruptness. Scanning electron microscopy (SEM) is conveniently used to evaluate the flatness of the hetero-interface, and transmission electron micros- copy (TEM) is used to observe the fine structure and the crystal imperfection of the hetero-interfa~e.~ At present these analytical techniques, however, are not

* Paper presented at ECASIA 89, Antibes, France, 23-27 October 1989.

0142-2421/90/0704337 $05.00 0 I990 by John Wiley & Sons, Ltd.

enough for the requirement of atomic-layer growth techniques.

In this paper, we present the developments of evalu- ation methods using sputter-assisted AES, SEM and TEM on AlGaAs/GaAs superlattices epitaxially grown by MOCVD.

AES with ion sputtering has been used mostly to evaluate the thickness and the composition of each superlattice layer and the abruptness of the hetero- i n t e r f a~e .~ .~ At present, there are two types of ion guns available: a static pressure type and a differential pumping type. It is considered that for surface analysis it is important to maintain the analysing chamber under ultrahigh vacuum as high as possible, and that the depth resolution may be deteriorated by the contami- nation of residual gas (e.g. O, , N2). Also, it is well known that the lower the Auger electron energy (E,) is, the shorter the inelastic mean free path (A) is. For example, 1 values for Al KLL (E = 1396 eV) and for A1 LVV (65 ev) are 2.2 nm and 0.4 nm, respectively.' Therefore, it is expected that the depth resolution is altered from 2.2 nm to 0.4 nm using low-energy Auger signals instead of high-energy signals. In addition, AES depth profiling is carried out with ion etching of inert gas (e.g. Ar, Xe). It is well known that the ion etching gives rise to the deterioration of depth resolution, i.e. the knock-on effect and the atomic mixing effect, and that these effects are reduced by lowering the ion energy. ' v 9

From these considerations, we examined three methods to improve the depth resolution of AES depth profiling : (1) A differential pumping-type ion gun is used instead

of a static pressure type to reduce the surface

Received 23 October 1989 Accepted 7 March 1990

434 K. KAJIWARA A N D H. KAWAI

adsorption of residual gas during AES measure- ment.

(2) Low-energy Auger signals-A1L3VV (65 eV), Ga M,M4M4 (55 eV), As M4NN (32 e-are used instead of high-energy Auger signals-A1 KLL (1396 eV), Ga LMM (1070 ev), As LMM (1228 eV)-to shorten the escape depths of Auger electrons.

(3) The Ar+ ion energy is lowered from 1 keV to 0.2 keV to reduce the ion sputtering-induced effects.

AES depth profiling can evaluate the thickness and the composition of each superlattice layer and the abruptness of the hetero-interface only one- dimensionally (i.e. on growth direction z). Moreover, it is a time-consuming method.

On the other hand, high-resolution SEM can easily evaluate the thickness of each layer and the flatness of the hetero-interface two-dimensionally by observing directly the cleaved surface. We confirmed the image resolution of SEM on the AlGaAsIGaAs superlattices grown by MOCVD and examined the flatness of the hetero-interface on cleaved surfaces x-z or y-z.

TEM sample preparation is difficult and takes a long time, but only TEM can evaluate simultaneously the crystalline quality and the flatness of the hetero- interface with the best image resolution among these evaluation methods. We confirmed the image resolution of TEM and examined the fine structure and the crystal imperfection of the hetero-interface on AlGaAsIGaAs superlattices grown by MOCVD. We can evaluate accurately AlGaAsIGaAs superlattices grown by MOCVD using these evaluation methods and then can understand perfectly the growth mechanism of MOCVD.

EXPERIMENTAL

Evaluation of AIGaAs/GaAs superlattices by AES

Dependence of depth resolution on analysing chamber pres- sure. When using a static pressure-type ion gun, the pumping work is stopped and the analysing chamber is leaked from 7 x lo-* Pa to 8 x Pa by Ar gas and then the sample surface is etched by Ar' ions during depth profile measurement. On the contrary, when using a differential pumping type, the ionization chamber in the ion gun is leaked from 1 x Pa to 2 x lo-' Pa, but the pumping work is continued and the analyzing chamber is maintained at 2 x Pa (the difference is

The analysis was performed in a JEOL JAMP-10s scanning Auger microprobe system, with an ion inci- dence angle of 55" to the sample normal. The measure- ment conditions were: electron accelerating voltage 5 kV; probe current 1 x A; modulation voltage 5 eVv_.Tp, Ar + ion energy 0.5 keV; etching rate N 0.5 nm min . Dependence of depth resolution on Auger electron energy. Measurement conditions of high-energy Auger signals were the same as those mentioned above, namely; electron accelerating voltage 5 kV; probe current 1 x A; probe diameter 30 pm; modulation voltage 5 eV,,. Auger signals used were A1 KLL, Ga LMM and As LMM.

On the other hand, measurement conditions of low- energy Auger signals were : electron accelerating voltage 2 kV; probe current 2 x A; modulation voltage 3 eV,, . Auger signals used were A1 LVV, Ga MMM and As MNN. In both cases, measurement conditions were optimized with great care, a differential pumping- type ion gun was used and Ar' ion etching conditions were: Ar+ ion energy 0.5 keV; icn current 1 x lo-' A; raster scanned area 1 x 1 mm2; etching rate 0.5 nm min-'.

Sample preparation

AIGaAslGaAs superlattices in the present work were grown by atmospheric pressure MOCVD in a vertical- type reactor.' The reactor was designed so that it was suitable for a quick exchange of the gas composition over the GaAs wafer. The source materials used were trimethyl gallium (TMG), trimethyl aluminium (TMA), 5% arsine/H, and H, as carrier gas. The growth rate of GaAs was 0.4 nm s-l and the growth temperature was 750°C. Superlattices were grown on a Cr-doped (100) GaAs substrate. Four samples were made for evaluation using sputter-assisted AES, SEM and TEM.

Sample no. 1 was an AlGaAsIGaAs multilayer film grown on a GaAs substrate. Sample no. 2 was a 3 nm thick A10~4,Gao~54As/3 nm thick GaAs superlattice grown on a GaAs substrate, with a 20 nm thick GaAs layer grown on the superlattice for oxidation protection. Sample no. 3 was a 5 nm thick AlAs/3 nm thick GaAs superlattice grown on a GaAs substrate, with a 10 nm thick GaAs grown on the superlattice.

Sample no. 4 was a 10 nm AlAs/lO nm GaAs super- lattice grown on a GaAs substrate, and we observed a (1 10) cleaved surface, which is the most preferred cleav- age plane of GaAs single crystal.

Dependence of depth resolution on Ar' ion energy. In this case, in order to obtain the best depth resolution, a dif- ferential pumping-type ion gun was used and the low- energy Auger signals were measured. Therefore, measurement conditions were the same as those used when low-energy Auger signals were measured (see pre- vious section).

The Ar' ion energy was varied as 1,0.5 and 0.2 keV, and the etching rate was varied as 0.46, 0.28 (1 monolayer) and 0.14 (3 monolayer) nm min-', respec- tively.

Observation of AIGaAs/GaAs superlattices by SEM and TEM

SEM observation conditions of sample no. 4 performed in an IS1 DS-130 were: accelerating voltage 25 kV; probe current 1 x lo-'' A; probe diameter 2 nm.

Also, SEM observation conditions of sample no. 3 performed in a JEOL JSM-890 were: accelerating voltage 15 kV; probe current 1 x lo-" A; probe dia- meter 0.7 nm.

TEM observation conditions of sample no. 3 per- formed in a JEOL JEM-200CX were : accelerating voltage 200 kV; spatial resolution 0.26 nm.

ANALYSIS OF AlGaAs/GaAs SUPERLATTICES 435

GaAs AI,Ga,-,As GaAs AIAs

4 6 n m 44.5 n m 4 3 n m 1 4 n m

RESULTS AND DISCUSSION

Sut

Evaluation of AIGaAs/GaAs superlattices by AES

. ( b )

. GaAs AIGaAs/GaAs 3 n m / 3 n m GaAs

Dependence of depth resolution on analysing chamber pres- sure. Measurement results of sample no. 1 using the static pressure method and the differential pumping method are shown in Fig. 1, where it is clearly seen that oxygen concentration is increased at the AlGaAs and AlAs layers using the static pressure method but oxygen is not detected in any of the layers using the differential pumping method. This is because the pumping work is stopped using the static pressure method and etching is

> I-

m z W I- z

-

- L 4 W a rn W (3 2 a

0 310 SPUTTERING TIME (rnin )

( a )

100 r 1

80 1 y;;m AlGaAs GaAs AlAs GaAs 4 5 n m 4 3 n m 1 4 n m

> 60 I-

m z W I- z

-

- 40

0 50 100 150

DEPTH ( n m )

Figure 1. Auger depth profiles of an AIGaAs/GaAs multilayer obtained by using: (a) a static pressure-type ion gun; (b) a differ- ential pumping-type ion gun.

performed so slowly that residual 0, molecules are adsorbed on the AlGaAs-etched surface, which is oxi- dized more easily than the GaAs surface; using the dif- ferential pumping method, the analysing chamber pressure is lowered from 8 x Pa to 2 x Pa during AES measurement and the pumping work is continued.

Oxygen contamination is diminished by replacing a static pressure-type ion gun with a differential pumping type. The main compositional profiles of the AlGaAsI GaAs multilayers, however, especially the abruptness of the hetero-interfaces, are not changed at all. The depth resolution is the same for the two types of ion guns if the etching conditions are well adjusted in both cases.

GaAs A I G a A s I G a A s 3 n m / 3 n m GaAs t > I -

m z W I- z

- -

I" KLL

" 50 100

0' 0

DEPTH ( n m )

0 0 50 100

DEPTH ( n m ) Figure 2. Auger depth profiles of a 3 nm AIGaAs/3 nm GaAs superlattice obtained using: (a) Al KLL, Ga L M M and As LMM; (b) Al L W , Ga MMM and As MNN.

436 K. KAJIWARA A N D H. KAWAI

10000

8000

> 6000

Dependence of depth resolution on Auger electron energy. Measurement results of sample no. 2 are shown in Fig. 2, where it is clearly seen that the depth resolution is improved to some extent by using low- energy Auger signals instead of high-energy signals, since the escape depths are shortened as the Auger elec- tron energies are lowered. In addition, the signal-to- noise ratio is drastically improved, because A1 LVV has a higher sensitivity than A1 KLL. Moreover, it is esti- mated that the irradiation damage to a sample surface is considerably reduced since a low accelerating voltage and a low probe current are optimum conditions when low-energy Auger signals are measured.

. ( c )

-

-

1000 I

1 8 00

As

> 600 c - m z W t

400 z

200

0 20 40 60 80 DEPTH ( n m )

Even if the low-energy Auger signals are used, the depth resolution is the sum of the inelastic mean free path (0.5 nm) and the degree of deterioration due to Ar+ ion etching (1.5 nm) corresponding to 0.5 keV ion energy, i.e. 2 nm; this value is regarded as the limiting resolution. Therefore, the chemical composition of each 3 nm thick layer may be measured inaccurately in the depth profile of Fig. 2(b). In fact, measured values of A10,,Ga,~,4As and GaAs are A1,,,,Ga0,,,As and A~,.,,Ga,~,,As, respectively.

Dependence of depth resolution on Ar+ ion energy. Measure- ment results of sample no. 3 are shown in Fig. 3, where

2000i >

m z W I-

t

f 1000

A I 0

OO0 20 40 60

DEPTH ( n m )

200 0' ' ' ' " " " I ' '

0 100

SPUTTERING TIME (min

Figure 3. Auger depth profiles of a 5 nm AIAs/3 nm GaAs superlattice obtained using different Ar+ ion energies: (a) 1 keV; (b) 0.5 keV; (c) 0.2 keV.

it is clearly seen that the lower the ion energy is, the better the depth resolution is, since the deterioration due to ion etching is diminished as the ion energy is lowered. In particular, the composition of the 3 nm thick GaAs layer depends strongly on the depth resolution, whereas that of the 5 nm thick AlAs layer depends little on the depth resolution. However, if the ion energy is lowered to ~ 0 . 2 keV, then the etching action stops and it is impossible to obtain the AES depth profile, since the etching rate is so slow that it competes with the adsorption rate under a pressure of 2 x Pa, which is the analysing chamber pressure differentially pumped.

From the results mentioned above, the best resolution was attained when lowenergy Auger signals were used and etching was performed at 0.2 keV Ar+ ion energy. Under this condition, the sum of the inelas- tic mean free path (0.5 nm) and the degree of deterio- ration due to ion etching (1 nm), i.e. 1.5 nm, is the limiting depth resolution of AES depth profiling.

Depth resoluth of AES depth proiiliog. To verify the AES depth resolution, a measurement result of a single quantum well AlAs/GaAs structure is shown in Fig. 4.2 From Fig. 4, the abruptness of the AlAs/GaAs hetero- interface is 1.6 nm according to the definition of a 1&W% transition width; this value coincides with the limiting depth resolution of sputter-assisted AES.

In addition, only the AES depth profiling can evalu- ate precisely the composition of a graded-index struc- ture. A measurement result of a parabolic graded Al,Ga,-,As layer grown by MOCVD is shown in Fig. 5. It is clearly seen that the composition of the graded Al,Ga, -,As layer resembles closely a parabolic curve.

ANALYSIS OF AIGaAs/GaAs SUPERLATTICES 437

- . GaAs structure. GaAs superlattice cleaved surface.

1 GaAs 1 AlAs 1 Cans 1 AlAs {GaA&b.

5 .0nm 2.5nm 2.3nm 2.7nm 2.5nm

, 6000 I-

v) z -

AS W I- Z -

4000 a a a

4 2000

W

W 0 3

t Surf ace 10 nm

DEPTH w

Figure 5. Auger depth profile of an AI,Ga, - 4 s parabolic graded layer.

Observation of AIGaAsIGaAs superlattices by SEM and TEM

A SEM observation result of sample no. 4 is shown in Fig. 6. It is clearly seen that the flatness of the hetero- interface is extremely good from the interface between the substrate and the initial epitaxial layer to the topmost layer. From Fig. 6, an AlAs layer has a dark contrast and a GaAs layer has a bright contrast, since the contrast of a SEM image is formed mainly by back- scattered electrons, which depend on the mean atomic number of materials. One should rely, however, on the sputter-assisted AES to analyse the composition of each layer. The AES depth profile of sample no. 4 is shown in Fig. 7, where the measurement conditions are those of the best resolution attainable. From Fig. 7, it is clearly seen that the dark contrast layer and the bright contrast layer are pure AlAs and pure GaAs, respectively.

Also, SEM images of sample no. 3, the AES data of which are shown in Fig. 3, are shown in Fig. 8. In Fig. 8(a), it is clear that the flatness of the hetero-interface

100 0' " " I ' ' " ' ' 0 50

SPUTTERING TIME (min 1

Figure 4. Auger depth profile of a single quantum well AIAs/ Figure 6. Scanning electron micrograph of a 10 nm AIAs/lO nm

438

8000

~ 6000 L

K. KAJIWARA A N D H. KAWAI

-

I - As

< 10000

0' " ' ' I " " ' I '

0 100 200

SPUTTERING TIME (min )

Figure 7. Auger depth profile of a 10 nm AIAs/lO nm GaAs superlattice.

has little fluctuation in the observed area, which is a (1 10) cleaved surface and includes the interface between the GaAs substrate and the epitaxial layer. On the other hand, in Fig. 8(b), the observed area is a (1 11) cleaved surface, which is a part of the epitaxial layer, and is projected such that the thickness of each layer is enlarged to a factor of sec (54.7") = 1.73. In addition, the contrast of AlAs in Fig. 8(b) is bright, but that in Fig. 8(a) is dark. It is supposed that the contrast of AlAs depends on the degree of preferential oxidation, and the sample surface in Fig. 8(b) is more oxidized than that in Fig. 8(a), since the former sample is inserted in the observation chamber for SEM after cleavage in air. Therefore, it is easily estimated that these contrast changes do not occur if the cleavage tool is attached under ultrahigh vacuum.

TEM observation results of sample no. 3, the AES data of which are shown in Fig. 3 and the SEM images of which are shown in Fig. 8, are shown in Fig. 9.' Figure 9(a) is a dark field image obtained by using (002) diffraction, and it is clearly seen that the AlAs layer has a bright contrast and the GaAs layer a dark contrast, and that the hetero-interface between AlAs and GaAs is abrupt to the order of one atomic layer. Figure 9(b) is a multi-beam interference lattice image. It is seen that the crystalline state of the AlAs/GaAs superlattice grown by MOCVD is perfect.

CONCLUSIONS

These evaluation techniques-sputter-assisted AES, SEM and TEM-have been examined for the analysis of AlGaAs/GaAs superlattices grown by MOCVD. We summarize as follows :

Figure 8. Scanning electron micrographs of a 5 nm AIAs/3 nm GaAs superlattice: (a) (110) cleaved surface and the interface between the GaAs substrate and the initial epitaxial layer; (b) (1 11 ) cleaved surface and a part of the epitaxial layer.

(1) The depth resolution of AES is attained to 1.5 nm by using a differential pumping ion gun, A1 LVV, Ga MMM and As MNN Auger signals and 0.2 keV Ar ' ion energy. Sputter-assisted AES can evaluate the thickness and chemical composition of each superlattice layer, the abruptness of hetero-interfaces and especially the profile of the compositionally graded layer.

(2) High-resolution SEM (e.g. spatial resolution 0.7 nm) can easily evaluate the flatness of the hetero- interface from the substrate to the uppermost surface of the superlattice by observing the cleaved surface, since the AlGaAs and GaAs layers are dis- tinguished by the signal contrast. Also, TEM (e.g. image resolution 0.2 nm) can evaluate the crystal imperfections of the superlattice by observing the ultrathinned cross-sectional sample, and evaluate the flatness of the hetero-interface in detail.

(3) Three analytical techniques-sputter-assisted AES, SEM and TEM--can evaluate accurately the super- lattice. It is confirmed from these evaluation results that the hetero-interface abruptness of AlGaAs f GaAs superlattices grown by MOCVD is of the order of one monoatomic layer.

ANALYSIS O F AIGaAs/GaAs SUPERLATTICES

(b)

439

Figure 9. Transmission electron micrographs of a 5 nm AIAs/3 nm GaAs superlattice, cross-sectional (110) surface: (a) (002) dark field image; (b) lattice image.

Acknowledgements

The authors would like to thank A. Mogami, Y. Sakai and T. Sekine (JEOL) for making the Auger system available, T. Sato (ISI), A. Ono and K. Ogura (JEOL) for the SEM observation and K. Ibe and H. Yotsumoto (JEOL) for the TEM observation.

This work was performed under the management of the R&D Association for Future Electron Devices as a part of the R&D Project of Basic Technology for Future Industries sponsored by the Agency of Industrial Science and Technology, MITI.

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