TRANSMISSION ELECTRON MICROSCOPY OF MULTILAYERED METAL AND SEMICONDUCTOR STRUCTURES

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TRANSMISSION ELECTRON MICROSCOPY OF MULTILAYERED METAL AND SEMICONDUCTOR

STRUCTURES

H. Oppolzer

To cite this version:

H. Oppolzer. TRANSMISSION ELECTRON MICROSCOPY OF MULTILAYERED METAL AND SEMICONDUCTOR STRUCTURES. Journal de Physique Colloques, 1987, 48 (C5), pp.C5-65-C5-74.

�10.1051/jphyscol:1987510�. �jpa-00226681�

JOURNAL DE PHYSIQUE

Colloque C 5 , supplbment a u n O l l , Tome 4 8 , novembre 1987

TRANSMISSION ELECTRON MICROSCOPY OF MULTILAYERED METAL AND SEMICONDUCTOR STRUCTURES

H. OPPOLZER

Siemens AG, Research Laboratories, O t t o - ~ a h n - R i n g 6 , 0-8000 Miinchen 83, F.R.G.

~esume: La microscopie electronique par transmission de sections transversales permet l'observation directe d'interfaces dans des structures en couche avec un pouvoir de resolution atteignant l'echelle atomique. Comme les sections montrent autant la direction verticafe, c'est M i r e la direction de la croissance de la couche que >a direction laterale, elles permettent, par exemple l'appreciation des irreg-ularites de l'interface, ains: que l'uniformite de la croissance des couches.

Plusieurs exemples son% presentes illustrant le genre, d'information qu'il est possible d'obtenir $e l'etude 051 TEM de structure de metal et de semiconducteur.

Les multicouches metalliques deposees par sputtering sont en rapport avec les procedes de metallisation pour la te,chnologie VLSI. Les structures semi-conducteurs sont des couches heteroepitaxiales deposees par MBE ou MOVPE.

Abstract: Transmission electron microscopy of crosssectional specimens allows to visualize the interfaces of layered structures directly with a spatial resolution down to an atomic scale. Since the crosssections display both the vertical direc- tion, i.e. the direction of layer growth and the lateral direction, they allow assessment of, e.g., abruptness of interfaces as well as lateral uniformity of film growth. A number of examples are presented to illustrate which type of information can be obtained by TEM characterization of both metal and semiconductor structures.

The metal multilayers were sputter deposited and are related to metallization system used in VLSI technology. The semiconductor structures are heteroepitaxial layers deposited by MBE or MOVPE.

The preparation of thin cross sections has opened new possibilities for the study of layered structures by transmission electron microscopy (TEM). The cross sections display both the lateral and the vertical direction, which is the direction of layer growth. Therefore, the microstructure of thin films can be studied directly as a function of growth direction, and the interfaces are viewed end-on. The latter is especially important for multilayer structures where the interface properties deter- mine all types of physical and electrical parameters.

Depending on the questions of interest the whole variety of techniques for TEM imaging and analytical TEM can be applied. For the study of multilayers with a thickness of the individual layers in the nanometer range, lattice imaging by high resolution electron microscopy is required. To illustrate which type of information can be obtained by TEM a number of examples was chosen for both metal and semicon- ductor structures. The metal multilayers were sputter deposited and include both amorphous and polycrystalline structures. They are related to metallization system used in VLSI silicon technology. The semiconductor structures are heteroepitaxial layers grown by molecular beam epitaxy (MBE) and metal organic vapour phase epitaxy

(MOVPE). In most cases they are related to semiconductor lasers.

2. Experimental

The preparation of cross sectional specimens has become a routine technique in recent years. The technique as it is used in the laboratory of the author is out-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987510

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lined shortly in Fig. 1 [l]. The TEM investigations were performed in a JEOL 200 CX microscope with a high resolution side entry goniometer at 200 kV beam voltage.

Standard bright field and dark field imaging, as well as high resolution imaging was applied.

Pig. 1 Preparation of cross sectional TEM specimens. a) glueing together two pieces (3mm X 3mm) face to face; b) cutting of slices (thickness of about 150 um) with a dicing saw (G glue); c) lapping and polishing of the slices down to a thickness of about 20 urn; d) argon ion etching at shallow angle; e) the specimen is etched until a hole appears in the center, and for easier handling it is glued onto a supporting ring. The thin edges of the hole are transparent to electrons (thickness < ^0.5 urn).

3. Multilayered metal structures 3.1. Tantalum-Silicon Multilayers

Because of its low resistivity double layers of refractory metal silicides like T&i2 and polysilicon are recently used as gate and interconnect material in VLSI circuits. Deposition of TaSi2 layers by co-sputtering from single element target results in the formation of alternating Si and Ta layers. By controlling the thick- ness of both types of layers the overall composition can be adjusted close to stoichiometry. Fig. 2a shows such a Ta-Si multilayer after deposition onto polysili- con. Due to the high atomic number the Ta layers appear dark and the Si layers bright. Both Ta and Si layers are amorphous after deposition. Annealing at 900°C in an inert ambient produces the low-resistive, crysta1line'~a-Si2 with a grain size of about 80 nm (Fig. 2b). The rough interface indicates some reaction between the T&i2 and the polysilicon which is desired [2]. To facilitate this reaction the native oxide on top of the polysilicon must be etched off in diluted hydrofluoric acid (HF

Fig. 2 TFN cross section of Ta-Si layer on polysilicon. a) Ta-Si multilayers after sputter deposition, b) crystalline TaSi2 after annealing at 900'C.

d i p ) p r i o r t o Ta-Si s p u t t e r i n g . CO-sputtering from s i n g l e element t a r g e t s has the advantage t h a t t h e type of t h e f i r s t l a y e r can be chosen a t w i l l . S t a r t i n g with a Ta l a y e r h e l p s t o reduce and/or break up t h e i n t e r f a c i a l oxide formed a f t e r t h e HF dip, and t h e r e f o r e supports a homogeneous r e a c t i o n . Ending with a S i l a y e r provides p a s s i v a t i o n with i t s n a t i v e oxide.

The Ta-Si m u l t i l a y e r s can be deposited with d i f f e r e n t thickness o f a double l a y e r . I n Fig. 2a t h i s t h i c k n e s s i s 10 nm. To achieve stoichiometry t h e thickness of the i n d i v i d u a l Ta and S i l a y e r s must be 3 nm and 7 nm assuming t h a t t h e d e n s i t i e s i n the t h i n f i l m s do n o t d e v i a t e much from t h e values f o r c r y s t a l l i n e bulk m a t e r i a l . The TEM c r o s s s e c t i o n s , however, showed equal thicknesses of 5 nm f o r both l a y e r s . Fig.

3 shows Ta-Si m u l t i l a y e r s with 3 nm and 20 nm double l a y e r t h i c k n e s s . For t h e 3 nm l a y e r i n Fig. 3a t h e i n d i v i d u a l l a y e r s a r e c l e a r l y resolved, b u t t h e Ta l a y e r s appear t h i c k e r than t h e S i l a y e r s . For t h e 20 nm l a y e r s i n Fig. 3b t h e thicknesses a r e 8 nm and 10.5 nm f o r t h e Ta and S i l a y e r s , r e s p e c t i v e l y . These r e s u l t s a s well a s measurements o f m u l t i l a y e r s with 5 nm and 40 nm double l a y e r t h i c k n e s s show t h a t t h e thickness of t h e Ta l a y e r s a r e always 2 nm l a r g e r than t h e nominal value whereas t h e S i l a y e r s a r e t h i n n e r by 2 nm. The i n t e r f a c e s appear sharp and t h e r e i s no i n d i c a t i o n o f a d i f f u s i o n region. One p o s s i b l e explanation is t h a t because of the high atomic number of Ta, both a p o s s i b l e d i f f u s i o n region and t h e Ta l a y e r show s i m i l a r dark c o n t r a s t and, t h e r e f o r e , cannot be d i s t i n g u i s h e d . Another model i s t h a t some S i i s d i s s o l v e d homogeneously i n t h e Ta l a y e r s . X-ray d i f f r a c t i o n a t t h e one- dimensional l a t t i c e of t h e m u l t i l a y e r s y i e l d s s e v e r a l peaks according t o i n c r e a s i n g o r d e r s of d i f f r a c t i o n . Computer simulation of t h e d i f f r a c t i o n p a t t e r n s u s i n g various models f o r i n t e r d i f f u s i o n support t h e second model mentioned above [3].

F i a . 3 Ta-Si m u l t i l a y e r s with 3 nm ( a ) and 20 nm ( b ) t h i c k n e s s of double l a y e r .

3.2 Layered aluminium f i l m s

M i n i a t u r i z a t i o n i n i n t e g r a t e d c i r c u i t technology r e q u i r e s a r e d u c t i o n of l i n e width of metal i n t e r c o n n e c t i o n s which l e a d s t o an i n c r e a s e of c u r r e n t d e n s i t y . R e l i a b i l i t y of t h e m e t a l l i z a t i o n i s determined by electromigration, i . e . material f l u x induced by c u r r e n t t r a n s p o r t . Local divergences i n t h e m a t e r i a l f l u x caused by inhomogenei- t i e s i n t h e g r a i n s t r u c t u r e r e s u l t i n t h e formation of h i l l o c k s , voids and l i n e i n t e r r u p t i o n s . To improve e l e c t r o m i g r a t i o n r e s i s t a n c e layered aluminium s t r u c t u r e s i n c l u d i n g one o r s e v e r a l i n t e r m e d i a t e l a y e r s of t r a n s i t i o n o r r e f r a c t o r y metals were suggested, and showed an i n c r e a s e of i n t e r c o n n e c t l i f e t i m e up t o two o r d e r s of magnitude [k]. The i n f l u e n c e of T i i n t e r m e d i a t e l a y e r s i n AI-Si(l%) l i n e s on e l e c t r o m i g r a t i o n behaviour i n combination with a TiN underlayer was s t u d i e d i n [S].

D e t a i l s of f i l m p r e p a r a t i o n by s p u t t e r d e p o s i t i o n and l i f e t i m e measurements were given i n [5]. TEM c r o s s s e c t i o n s o f two samples p r i o r and a f t e r annealing a t 450°C (30 min) a r e shown i n Fig. 4. The f i r s t sample c o n t a i n s 6 T i l a y e r s of 5 nm thick- n e s s , i n t h e second sample only one 20 nm t h i c k T i l a y e r was included. I n both cases t h e t h i c k n e s s of t h e t o t a l l a y e r s t r u c t u r e was 1 um. After d e p o s i t i o n t h e T i l a y e r s of both samples a r e continuous (Fig. h a , c ) . I n c o n t r a s t t o t h e 5 nm l a y e r s which a r e amorphous, t h e 20 nm l a y e r is c r y s t a l l i n e . Due t o nonplmlar i n t e r f a c e s t h e Ti l a y e r s do n o t appear with c o n s t a n t thickness i n t h e c r o s s s e c t i o n . During annealing i n t e r m e t a l l i c compounds a r e formed. X-re d i f f r a c t i o n p a t t e r n s showed t h e phase A13Ti f o r t h e 5 nm l a y e r s . I n t h e sample ^{h ,I} t h e 20 nm l a y e r both an Al-rich phase, i . e . A13Ti, and a small f r a c t i o n of a T i - r i c h phase (Ti3A1) was found. The 5 nm

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Fig. 4 Layered Aluminium s t r u c t u r e s . ( a ) , ( b ) s i x 5 nm t h i c k T i i n t e r m e d i a t e l a y e r s ( o n l y 4 a r e shown); ( c ) , ( d ) one 20 nm t h i c k T i i n t e r m e d i a t e l a y e r . Samples i n Fig.

( a ) and ( c ) a r e a f t e r d e p o s i t i o n , sam- p l e s i n Fig. ( b ) and (d) a f t e r annealing a t 450°C.

l a y e r s a r e no more continuous a f t e r annealing b u t c o n s i s t of s h e e t s of i n d i v i d u a l A13Ti g r a i n s ( F i g . 4 b ) . The 20 nm i n t e r m e d i a t e l a y e r , however, y i e l d s a continuous l a y e r of i n t e r m e t a l l i c compounds ( F i g . 4 d ) . The r e s i s t i v i t y of t h e l a y e r s i n c r e a s e s s l i g h t l y when compared t o A1-Si(l%) due t o consumption of A 1 by compound formation.

Compared t o an A l - S i ( l % ) - T i ( 0 , 2 % ) a l l o y , t h e l i f e t i m e of t h e f i r s t sample ( s i x 5 nm T i l a y e r s ) was i n c r e a s e d by about an o r d e r of magnitude, t h a t of t h e second sample

(one 20 nm T i l a y e r ) by about two o r d e r s [ 5 ] . The T i l a y e r s i n f l u e n c e t h e A 1 micro- s t r u c t u r e such t h a t a more uniform g r a i n s i z e d i s t r i b u t i o n is obtained. This i s known t o improve e l e c t r o m i g r a t i o n r e s i s t a n c e . Furthermore, t h e continuous i n t e r - m e t a l l i c l a y e r of t h e second sample r e p r e s e n t s a d i f f u s i o n b a r r i e r which e f f e c t i v e l y blocks c r a c k growth through t h e f i l m during c u r r e n t stress [Q]. More d e t a i l e d r e - s u l t s w i l l be p r e s e n t e d i n another paper by t h e a u t h o r s of Ref. 5.

Multilayered A1-MO and A l - N i f i l m s with l a y e r t h i c k n e s s e s i n t h e nanometer range show i n t e r e s t i n g e l e c t r i c a l p r o p e r t i e s [ 6 ] . Such f i l m s were prepared by s e q u e n t i a l s p u t t e r d e p o s i t i o n . The l a y e r e d s t r u c t u r e i s c l e a r l y seen i n Fig. 5 f o r both c a s e s , with t h e A 1 l a y e r s appearing b r i g h t and t h e MO, r e s p . N i , l a y e r s dark. The o r i g i n of t h e v e r t i c a l s t r i p e s i n Fig. 5b is n o t c l e a r . Dark f i e l d images showed t h a t both m u l t i l a y e r s c o n t a i n A 1 g r a i n s extending over s e v e r a l l a y e r s depending on t h e mater- i a l combination and d e p o s i t i o n c o n d i t i o n s . Measurement of t h e mean g r a i n s i z e of A 1 on Plan view specimens, however, y i e l d e d v a l u e s c l o s e t o t h e A 1 l a y e r t h i c k n e s s . E l e c t r i c a l c o n d u c t i v i t y i s determined p r i m a r i l y by t h e A 1 Layers. Depending on t h e

Fig. 5 a ) A1-MO m u l t i l a y e r (monolayer t h i c k n e s s : 1 nm A l , 1 nm MO); b ) A 1 - N i multi- l a y e r (monolayer t h i c k n e s s : 2 nm Al, 0.5 nm Ni)

A 1 l a y e r t h i c k n e s s and t h e r e f o r e mean g r a i n s i z e , p o s i t i v e and n e g a t i v e temperature c o e f f i c i e n t s of r e s i s t i v i t y were found and could be explained by a model assuming e f l e c t i o n of t h e conductive e l e c t r o n s a t t h e g r a i n boundaries [6].

4. Multilayered semiconductor s t r u c t u r e s

4.1. Imaging techniques f o r h e t e r o e p i t a x i a l m u l t i l a y e r s

During c h a r a c t e r i z a t i o n of h e t e r o e p i t a x i a l m u l t i l a y e r s of l a t t i c e matched semicon- d u c t o r s by TEM, u s u a l l y t h e l a y e r thicknesses, abruptnes o f t h e i n t e r f a c e s , and compositional v a r i a t i o n s i n t h e l a y e r s a r e of i n t e r e s t . The technique most commonly a p p l i e d i s dark f i e l d (DF) imaging using t h e i n t e n s i t y of t h e (200) r e f l e c t i o n [7].

For t h e diamond l a t t i c e ( l i k e S i and Ge) t h i s r e f l e c t i o n is forbidden. For GaAs the s t r u c t u r e f a c t o r is small s i n c e t h e atomic s c a t t e r i n g f a c t o r s of Ga and A s a r e n e a r l y i d e n t i c a l . For Gal-,AlXAs t h e s t r u c t u r e f a c t o r , o f t h i s r e f l e c t i o n i s

where fA1 and fGa? i . e . t h e atomic s c a t t e r i n g f a c t o r s of A 1 and Ga, d i f f e r consider- ably. The intensity of t h e (200) r e f l e c t i o n 1200 = IF^^^^^ i s proportional t o x2.

Therefore, t h e GaAs l a y e r s i n Figures 7 t o 10 always appear dark, whereas t h e GaAlAs l a y e r s show b r i g h t c o n t r a s t . Changes i n t h e A 1 content of Ax 2 0.05 a r e v i s i b l e . From changes i n c o n t r a s t q u a n t i t a t i v e r e s u l t s f o r t h e composition can be derived [8]. However, t h i s is n o t s t r a i g h t forward s i n c e t h e i n t e n s i t y of t h e (200) r e f l e c - t i o n a l s o depends on specimen thickness and t h e d e v i a t i o n from Bragg o r i e n t a t i o n . Due t o t h e small s i z e of t h e o b j e c t i v e a p e r t u r e necessary t o s e l e c t t h e (200) r e f - l e c t i o n , t h e s p a t i a l r e s o l u t i o n i s l i m i t e d t o 0.5 nm. This i s , however, s u f f i c i e n t i n many c a s e s .

Recently a technique was proposed which does not r e q u i r e t h e p r e p a r a t i o n of t h i n c r o s s s e c t i o n s a s described i n Fig. I. Samples a r e prepared by cleavage i n both (110) and ( i 1 0 ) planes and a r e mounted such t h a t t h e cleaved 90" edge i s perpendicu- l a r t o t h e e l e c t r o n beam [g]. I n t h e v i c i n i t y of t h e cleaved edge t h e sample can be t r a n s m i t t e d and t h e layered s t r u c t u r e s a r e viewed end-on a s i n t h e c a s e of cross s e c t i o n s . By e x c i t a t i o n of s t r o n g r e f l e c t i o n s t h i c k n e s s contours appear i n b r i g h t f i e l d images. The p o s i t i o n of t h e thickness f r i n g e s changes with composition of the l a y e r . Because of t h e w e l l defined geometry of t h e 90" edge q u a n t i t a t i v e a n a l y s i s is p o s s i b l e by measuring t h e thickness f r i n g e s . The s p a t i a l r e s o l u t i o n i n t h e d i r e c t i o n of i n t e r e s t normal t o t h e l a y e r s is s i m i l a r a s f o r t h e (200)-DF method. Planar i n t e r f a c e s a r e required f o r t h i s technique.

For c h a r a c t e r i z a t i o n of s u p e r l a t t i c e s with l a y e r t h i c k n e s s e s of a few atomic planes t h e s p a t i a l r e s o l u t i o n of t h e described techniques i s not s u f f i c i e n t , and high r e s o l u t i o n e l e c t r o n microscopy (HREM) has t o be a p p l i e d . Usually c r o s s s e c t i o n s a r e imaged i n [l101 p r o j e c t i o n , and f o u r (111) r e f l e c t i o n s and two (200) r e f l e c t i o n s

GaAs

G a A s

Fig. 6 High r e s o l u t i o n e l e c t r o n micrograph i n [110] p r o j e c t i o n of G a A s - A 1 A s multi- l a y e r deposited by MBE.

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contribute to the image. Only certain combinations of specimen thickness t and defocus Af of the objective lens, however, yield sufficient contrast between layers of different composition. Suitable combinations of t and Af can be found by computer simulation of HREM images. Fig. 6 shows part of a GaAs-A1As multilayer deposited by MBE [10]. The values of t and Af were such that the AlAs layer exhibits additional white spots. These "half spacings" allow to distinguish the layers when the differ- ence in composition, i.e. Ax, is large. For low Ax (e.g. Ax = 0.3) the contrast is weak. The reason for this is that in [l101 projections the two (200) reflections are dominated by the four strong (111) reflections which are not sensitive to Ax. In contrast to this, the [l001 projection allows to let only the four (200) reflections contribute to the image. These are very sensitive to Ax and the resulting HREM images, therefore. provide good contrast between layers also in the case of small difference in composition [l11 . Furthermore, the deviation of the virtual interface in HREM-images from the real interface is smaller in [l001 projec-tions than in [l101 projections [l2 . Since the lattice spacings imaged in [l001 projection are smaller than in [l10

3

In addition to imaging techniques, the methods of analytical electron microscopy can be used to characterize multilayers. Energy-dispersive x-ray spectrometry allows to measure directly the composition of layers when cross sections are analyzed with a very fine electron probe [l41 . The spatial resolution, however, is limited to seve- ral nm.

4.2 GaAs - GaAlAs Multilayers

Multilayers in the system GaAs - GaAlAs are naturally lattice matched since the lattice parameters of GaAs and AlAs differ only slightly: Aa/a = 1.4-10-~. Both MBE and MOVPE are used to grow multilayer structures for various applications related to new photoelectronic and electronic devices. A few examples were chosen to illustrate problems during film growth.

Fig. 7 GaAs-G% A1 As multiquantum well structures grown by MBE; (200)-DF images.

a) ~ o m p o s i t i o n a ~ ~ o s ~ ~ ? 1 a t i o n s in GaAlAs barriers, b) MQW structure on top of thicker GaAlAs layer with rough surface (arrow).

Fig. 7a shows part of a multi quantum well (MQW) structure grown by MBE. Both the GaAs wells and the GaAlAs barriers have a thickness of 14 m. The abruptness of the interfaces is at or below the resolution limit of (200)-DF images of 0.5 nm. The GaAlAs barriers show striations normal to growth direction with a period of about 2 nm. The striations correspond to compositional oszillations induced by substrate rotation and can be attributed to small variations of the flux profile of the group I11 elements over the substrate area [15]. MBE growth of thicker GaAlAs layers can lead to the formation of rough surfaces. The interface indicated by an arrow in Fig.

7b is the surface of such a 0.35 um thick Gag 7A10 As layer showing undulations with differences in height up to 15 nm. An incrgased content results in stronger undulations. No clear dependence of the undulations from growth temperature as described in [l61 was found here [IT]. The GaAs - GaAlAs MQW structure on top of the thick GaAlAs layer levels the undulations nearly completely (Fig. 7b). Most of the leveling which is caused primarily by the GaAs layers, is accomplished after the first few layers.

Fig. 8 GaAs-Ga0.7A10e3As MQW s t r u c t u r e grown by MOVPE with non uniform A 1 content i n b a r r i e r s .

MOVPE allows t o grow m u l t i l a y e r s with s i m i l a r abrupt i n t e r f c e s a s MBE when t h e gas f l u x e s a r e c o n t r o l l e d properly. The i n d i v i d u a l l a y e r s of t h e MQW s t r u c t u r e i n Fig. 8 which was grown by MOVPE, show sharp i n t e r f a c e s . For t h i s s p e c i a l sample t h e con- t r a s t of t h e G a A l A s b a r r i e r l a y e r s , however, i s not uniform. The dark band i n the lower p a r t of t h e b a r r i e r l a y e r i n d i c a t e s a region where t h e A 1 content i s reduced by more than 502. The m u l t i l a y e r s of Fig. 9 were grown by MOVPE a t low growth r a t e . The t h i c k n e s s of t h e s i n g l e l a y e r s is about 4 m. The m u l t i l a y e r of Fig. 9a was grown with growth i n t e r r u p t i o n s a t t h e i n t e r f a c e s whereas t h a t of Fig. 9b was grown without i n t e r r u p t i o n s . I n both cases s i m i l a r sharp i n t e r f a c e s , were obtained with an abruptness o f t h e o r d e r of t h e r e s o l u t i o n l i m i t of (200)-DF images [18].

Fig. 9 GaAs-G%..,Al0 3 A s m g l t i l a y e r s grown by MOVPE with ( a ) and without ( b ) growth i n t e r r u p t i o n s a t ~ n c e r f a c e s ; (200) -DF images. The g r a n u l a r c o n t r a s t is due t o d e f e c t c l u s t e r s near t h e s u r f a c e induced by i o n t h i n n i n g .

Zink d i f f u s i o n is known t o induce disordering of G a A s - G a A l A s multiquantum well s t r u c t u r e s [lg]. A GaAs - GaAlAs s u p e r l a t t i c e was grown by MBE, and an S i N 4 mask forming a s t r i p e p a t t e r n was used f o r l o c a l Zn d i f f u s i o n [20]. Fig. 10 sl?ows t h e t r a n s i t i o n region between t h e disordered zone and t h e i n t a c t s u p e r l a t t i c e i n t h e v i c i n i t y o f t h e Si3N4 mask edge. Due t o underdiffusion t h e disordered zone extends 0.5 pm under t h e Si3N4 mask edge (Fig. 1 0 a ) . The width of t h e t r a n s i t i o n region i n l a t e r a l d i r e c t i o n i s only 0.2 pm ( F i g . l o b ) . Such low values can be explained by t h e s t e p Zn concentration g r a d i e n t of t h e d i f f u s i o n f r o n t [21].

4.3 GaInAs-AlInAs m u l t i l a y e r s

The (GaInA1)As system is of g r e a t i n t e r e s t f o r optoelectronic a p p l i c a t i o n s since i t can be a p p l i e d t o t h e wave l e n g t h range o f 1.3 t o 1 . 6 pm, but avoids the'problems of phosphorus i n MOVPE and MBE systems. To grow l a y e r s l a t t i c e matched t o t h e InP s u b s t r a t e , p r e c i c e c o n t r o l of t h e composition i s necessary: Gao.471no_ 3 A ~ and A10.481n0.52AS. Fig. l l a shows a MQW s t r u c t u r e grown by MBE with a well t h i a n e s s of 2.5 nm. Growth and luminescence p r o p e r t i e s were described i n [22]. The AlInAs bar- r i e r l a y e r s appear dark s i n c e t h e mean value of t h e atomic s c a t t e r i n g f a c t o r s of A 1

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Fin. 10 Zn diffusion induced disordering of G ~ A S - G ~ ~ . ~ A ~ ~ 3~ superlattice. a) region around Si3N4 mask edge, b) disordered-ordered transi'tzon region; (200)-DF images.

and In is close to that of Ga. The GaInAs wells, however, appear bright due to the larger atomic scattering factor of In. The interfaces are abrupt within the reso- lution of (200)-DF images (0.5 m ) . The MQW structure of Fig. llb was grown by MOVPE and also exhibits sharp interfaces [23]. Nucleation of dislocations at the inter- faces of the MQW was observed when lattice mismatch occured due to deviation from optimum composition. By c reful optimization of the flow rates, however, a lattice a

mismatch of Aa/a 2 4 10- could be obtained.

Fin. 11 GaInAs-AlInAs MQW structure grown by MBE (a) and MOVPE (b); (200)-DF images

4.4 Non lattice matched heteroepitaxial layers

Superlattices in the system Ge-Si are not lattice matched. For layer thicknesses below a critical value no misfit dislocations are generated, and misfit is accommo- dated by elastic strain. An overview on Ge-Si strained-layer superlattices is given in this volume 1241. As in the case of 111-V heteroepitaxial multilayers HREM allows to characterize the interfaces [25]. However, elastic relaxations of the strained layers in the thin cross sections has to be considered [26]. Besides such superlat- tices lattice mismatched heteroepitaxial layers such as GaAs on Si are gaining increasing interest. As another example with a similar high misfit of Aaja = 4%, Fig. 12 shows an MBE grown GaAs layer on InP substrate. In the bright field image of Fig. 12a a high density of dislocations is visible threading from the interface through the GaAs layer. The misfit dislocations at the interfaces are difficult to resolve since their spacing of several nm is similar to that of ~ o i r e fringes which appear when two crystals with different lattice parameters are projected on top of each other. In HREM images taken in [l101 projection, however, the misfit disloca- tions can be identified by additional lattice planes. In Fig. 12b two such dislo- cations are indicated by marking the terminating lattice plane. By drawing a Burgers circuit around the dislocation the projections of the Burgers vectors onto the (110)

Fig. 12 GaAs layer grown by MBE on InP. a) bright field image showing threading dislocations in the GaAs, b) HREM image of interface showing two misfit disloca- tions.

image plane was found to be bp = 1/4 <112> which corresponds to 60" dislocations.

The mean distance of the misflt dislocations of 5 nm agreed well with the nominal valw of 6 nm necessary for complete misfit accommodation.

4.5 Doping layers

Doping concentrations are usually too low to produce contrast effects in TEM images.

For very high concentrations as in &-function doped layers, however, contrast was observed. Such a structure 6-doped with Sb was grown by MBE in the following way [27] : After growth of a buffer layer the sample was cooled to room temperature.

Then. Sb was evaporated to submonolayer coverage and coated with 2-3 nm Si. By reheating to 700" C the layer was recrystallized and epitaxial growth was continued.

In the bright field image of Fig. 13a the Sb doped layer is seen as a dark band at a depth of 18 nm having a width of 1.5 to 2 m. The specimen was tilted such that diffraction contrast is weak, and the dark contrast of the Sb doped layer is mostly due to increased scattering by the heavy Sb atoms. The dark band at the wafer sur- face stems from the native oxide. No defect clusters or precipitates could be found in the MBE layers. The perfect alignment of both the 6-layer and the S i layer on top of it is demonstrated in the HFlEM image of Fig. 13b. The position of the 6-layer is indicated since its contrast due to increased scattering is too weak. The perfect

position of 6-layer

Fig. 13 6-function Sb doped layer in MBE grown Si. a) bright field image showing

&-doped layer, b) HREM image showing perfect alignment of Si lattice.

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alignment o f t h e & - l a y e r s u g g e s t s t h a t a l l o r most o f t h e Sb i s e n c o r p o r a t e d on l a t t i c e s i t e s . Assuming t h e t o t a l amount o f Sb ( 5 10'3 cm-2) t o be uniform d i s r i b u t e d o v e r a l a y e r o f 2 nm t h i c k n e s s , y i e l d s a c o n c e n t r a t i o n o f 2.5 . ^{1 0 $8}

cm-5 which is f a r above t h e bulk s o l i d s o l u b i l i t y at t h e growth temperature o f 700°

C. Tunneling s p e c t r o s c o p y and t r a n s p o r t measurements gave e v i d e n c e f o r quantum confinement o f t h e e l e c t r o n i c charge i n t h e l a y e r and t h u s confirmed t h e narrow width [27] .

5. Conclusions

The examples p r e s e n t e d demonstrate t h a t TEM o f t h i n c r o s s s e c t i o n s r e p r e s e n t s a powerful t o o l f o r t h e c h a r a c t e r i z a t i o n o f m u l t i l a y e r e d s t r u c t u r e s . Both s t r u c t u r a l and c o m p o s i t i o n a l i n f o r m a t i o n i s obtained. Depth p r o f i l i n g techniques l i k e Ruther- f o r d b a c k s c a t t e r i n g and Auger e l e c t r o n s p e c t r o m e t r y p r o v i d e i n f o r m a t i o n on chemical composition on a more q u a n t i t a t i v e b a s i s , b u t do n o t reach t h e high depth r e s o l u t i o n o f TEM ( 5 0 . 5 m ) which i s n e c e s s a r y t o s t u d y m u l t i l a y e r s with s m a l l t h i c k n e s s e s . From HREM images t h e atomic s t r u c t u r e o f i n t e r f a c e s can be d e r i v e d . When s t u d y i n g f e a t u r e s l i k e d i s l o c a t i o n s o r growth s t e p s , however, t h e i r p r o j e c t i o n must be p a r a l - l e l t o t h e e l e c t r o n beam.

Acknowledgements The a u t h o r i s indepted t o H. Cerva, C . F r u t h , V . Huber, and S.

S c h i l d f o r a s s i s t a n c e with t h e TEM work, t o W. Eckers f o r t e c h n i c a l a s s i s t a n c e , and t o L. R e i d t f o r t h e photographic work. Valuable d i s c u s s i o n s w i t h K. A l a v i , M.

Druminski. R.W.H. Engelmann, K . Hieber, L. K o r t e , P. Kiicher. G. Roska, and E. Veu- h o f f a r e g r a t e f u l l y acknowledged.

References

1 H. Rehme, H. Oppolzer, Siemens Forsch. U . Entwickl. Ber. a (1985) 193 2 D.. Pawlik, H. Oppolzer, T. Hillmer. J . Vac. S c i . Technol. (1985) 492 3 H. Meyerheim, H. Gibel, t o be published

4 J . K . Howard, J.F. White, P.S. Ho, J. Appl. Phys. 5 (1978) 4083

5 P. Kiicher, G. Rijska, Proc. I n t . Symp. on Trends and New A p p l i c a t i o n s i n

tl

8

1

[g] H. ~ a k i b a y a s h i , F. Nagata, Jap. J. Appl. Phys. 24 (1985) L905, and 3 (1986) 1644

10 K. Ploog, Ann. Rev. Mater. S c i . 2 (1981) 171 11 Y. Suzuki, H. Okamoto, J . Appl. Phys. (1985) 3456.

12 C. J.D. Hetherington e t . a l . , Mat. Res. Soc. Symp. Proc. Vol. 37 (1985) 4 i 13 W.M. S t o b b s , t h i s volume

14 J . F . Bullock, J.M. Titchmarsh, C.J. Humphreys, Semicond. S c i . Technol. 1

/ l

[l51 K. A l a v i , P.M. P e t r o f f , W.R. Wagner, A . Y . Cho, J. Vac. S c i . Technol K ( l 9 8 3 ) 146

[l61 M.R. T a y l o r , M. Hockly, D.A. Andrews. G . J . Davies. I n s t . Phys. Conf. S e r . No.76: S e c t i o n 7 (1985) 295

'18: 17' :lg-

20 121:

-22-

K. A l a v i , I . Shidlovsky, p r i v a t e communication E. Veuhoff, H. Baumeister, t o be published

W.D. L a i d i g e t . a l . , Appl. Phys. L e t t . 3 (1981) 776

K. A l a v i , R.W.H. Engelmann, C.-L. S h i e h , p r i v a t e communication

K. I s h i d a , T. Ohta, S. Semura, H. Nakashima, Jap. J. Appl. Phys. 24 (1985) L620 W. S t o l z , K. Fujiwara, L. T a p f e r , H. Oppolzer, K . Ploog, I n s t . Phys. Conf.

S e r . No. 74: Chapter 3 (1985) 139

23 M. Druminski, R. Gessner, t o be published 24 H. Brugger. t h i s volume

25 R. H u l l , J . M . Gibson, J . C . Bean. Appl. Phys. L e t t . 46 (1985) 179 26 J . M . Gibson. R. H u l l . J . C . Bean, Appl. Phys. L e t t . 46 (1985) 649

27 H.P. Z e i n d l . T. Wegehaupt. I. E i s e l e . H. Oppolzer, H. R e i s i n g e r , G. Tempel,

t ⁱ