STUDIES OF SEMICONDUCTORS WITH ELECTRON AND ION BEAMS

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STUDIES OF SEMICONDUCTORS WITH ELECTRON AND ION BEAMS

D. Wittry

To cite this version:

D. Wittry. STUDIES OF SEMICONDUCTORS WITH ELECTRON AND ION BEAMS. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-855-C2-860. �10.1051/jphyscol:19842196�. �jpa-00223872�

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Colloque C2, supplgment au n02, Tome 45, f6vrier 1984 page C2-855

STUDIES OF SEMICONDUCTORS WITH ELECTRON AND ION BEAMS D.B. W i t t r y

Departments o f Materials Science and EZectricaZ Engineering, University of Southern C a l i f o r n i a , U n i v e r s i t y Park MC 0241, Los AngeZes, CA 90089-0241, U.S.A.

Resume - On passe en revue l e s experiences men&es a l ' a i d e des e l e c t r o n s e t des f a i s c e a u x de p a r t i c u l e s pour l f @ t u d e des semiconducteurs men@e "The University of Southern C a l i f o r n i a " .

Abstract - Experiments involving t h e use of e l e c t r o n and beams f o r the study of semiconductors t h a t have been c a r r i e d out a t the University of Southern C a l i f o r n i a a r e reviewed.

Introduction

Electron microprobe instruments have been used f o r s t u d i e s of semiconductors f o r more than 25 y e a r s . The e a r l y work i n t h i s f i e l d u t i l i z e d t h e c a p a b i l i t y of t h e e l e c t r o n probe microanalyzer t o perform a l o c a l elemental a n a l y s i s , f o r example i n s t u d i e s of t h e d i f f u s i o n of z i n c i n t o gallium a r s e n i d e and t h e d i f f u s i o n of a r s e n i c i n t o germa- nium and germanium regrown with a1 uminum.1 However, because of t h e 1 imi t e d sensi t i - v i t y f o r d e t e c t i o n of i m p u r i t i e s i n semiconductors, e f f o r t s were made in t h e e a r l y 1 9 6 0 ' s t o obtain information from o t h e r types of s i q n a l s t h a t r e s u l t from t h e e l e c - t r o n bombardment of semiconductors. The most useful of t h e many s i g n a l s a v a i l a b l e have been l i g h t s i g n a l s such as cathodoluminescence and specimen c u r r e n t o r e l e c t r o n beam induced c u r r e n t s .

With t h e development of instruments f o r secondary ion mass spectrometry many problems involving measurement of i m p u r i t i e s a t concentrations too low t o be detected by e l e c t r o n probe microanalysis could be addressed. Moreover, t h i s technique provided t h e c a p a b i l i t y of high s p a t i a l r e s o l u t i o n i n one dimension t h a t i s required in some a r e a s of semiconductor research.

In t h i s paper, examples a r e given from t h e research done a t USC which demonstrate t h e c a p a b i l i t i e s of e l e c t r o n and ion beams i n research on semiconductors.

Cathodoluminescence

Cathodoluminescence s i g n a l s have been useful t o provide t h e following types of information on semiconductors:

1 ) Local d e f e c t s including g r a i n boundaries, p-n j u n c t i o n s , p r e c i p i t a t e s or i n c l u s i o n s by CL images

2) Variations i n impurity concentration by CL images

3 ) Variations i n t h e energy band gap with composition i n mixed c r y s t a l s by CL s p e c t r a

4 ) S t a t e s i n t h e energy gap by low temperature CL s p e c t r a

5 ) S t a t e s i n t h e energy gap by i n f r a r e d modulated cathodoluminescence

6) Excess c a r r i e r d i f f u s i o n lengths by t h e voltage dependence of CL i n t e n s i t y . Figures 1-5 i l l u s t r a t e cases 1 , 3, 4 , 5 and 6.

Figure 1 i l l u s t r a t e s t h e c a p a b i l i t y of scanning CL images t o reveal local d e f e c t s . In t h i s c a s e , i t can be seen by comparison of t h e CL image ( a ) and t h e backscattered e l e c t r o n .image ( b ) t h a t twin boundaries do not.resu1 t . i n a ,decreased CL i n t e n s i t y . This i n d i c a t e s t h a t twin boundaries a r e e l e c t r i c a l l y i n a c t i v e a s i s expected.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19842196

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Fig. 1 - Cathodoluminescence image ( a ) and backscattered e l e c t r o n image ( b ) o f Raytheon ZnSe. By comparison o f t h e two images i t i s p o s s i b l e t o i d e n t i f y g r a i n boundaries (network o f black l i n e s i n a), t w i n boundaries (arrows i n b), surface scratches ( r i n a ) , i n c l u s i o n s (a i n a) and s u r f a c e p a r t i c l e s (o i n a and b ) . Figure 2 i l l u s t r a t e s t h e c a p a b i l i t y o f CL spectra i n determining t h e v a r i a t i o n o f energy band gap w i t h composition. I n t h i s case, i t i s i n t e r e s t i n g t o n o t e t h a t peaks were observed i n t h e spectrum t h a t correspond t o energies g r e a t e r than t h e band-gap energy.

Fig. 2 - Energy o f t h e peaks i n the cathodoluminescence s p e c t r a vs c r y s t a l composi- t i o n taken a t 30 and 300 K w i t h an electron-beam v o l t a g e o f 50 k ~ . ~

F i g u r e 3 i l l u s t r a t e s t h e c a p a b i l i t y o f l o w temperature CL s p e c t r a (and a l s o l o w temperature photoluminescence s p e c t r a ) t o r e v e a l s t a t e s i n t h e energy band-gap.

ENERGY ( e V )

1.5 1.4 1.3 1.2

I I I I I

WAVELENGTH ^(%)

F i g . 3 - Comparison o f cathodoluminescence spectrum a t 30 K (dashed l i n e s ) and photoluminescence spectrum a t 4.2 K ( s o l i d l i n e s ) f o r a specimen o f GaAs:Cr h e a t t r e a t e d i n Hz f o r 1/2 h a t 8 0 0 " c . ~

F i g u r e 4 ill u s t r a t e s t h e c a p a b i l i t y o f i n f r a r e d induced cathodoluminescence (and i n f r a r e d induced changes i n specimen c u r r e n t ) t o s t u d y s t a t e s i n t h e energy band gap. T h i s experiment i s b a s i c a l l y a c o n t a c t l e s s method f o r s t u d y i n g p h o t o c o n d u c t i - v 1 t y .

F i g . 4 - R e l a t i v e i n c r e a s e i n i r - i n d u c e d cathodoluminescence i n t e n s i t i e s a I / I and r e l a t i v e specimen c u r r e n t i n c r e a s e a I s / I S as a f u n c t i o n o f w a v e l e n ~ ~ t h . 4

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F i g u r e 5 i l l u s t r a t e s t h e c a p a b i l i t y o f d e t e r m i n i n g t h e d i f f u s i o n l e n g t h o f excess c a r r i e r s by t h e v o l t a g e dependence o f c a t h o d o l uminescence i n t e n s i t y .

F i g . 5

-

5

E l e c t r o n beam modulated r e f l e c t a n c e

I n t h e i n v e s t i g a t i o n s o f semiconductors by e l e c t r o n bombardment i t was found t h a t t h e r e f l e c t a n c e was m o d i f i e d by bombardment w i t h an e l e c t r o n beam. T h i s p r o v i d e s an i n t e r e s t i n g p o s s i b i l i t y o f d e t e r m i n i n g c r i t i c a l p o i n t s i n t h e band s t r u c t u r e . An example o f t h e work done on t h i s s u b j e c t 6 i s shown i n F i g . 6.

F i g . 6 - T y p i c a l modulated r e f l e c t a n c e spectrum o f g a l l i u m a r s e n i d e a t 1 kHz, 10 kV, and 0.47 A cm-2.

E l e c t r o n beam induced c u r r e n t s (EBIC)

E l e c t r o n beam induced c u r r e n t s a r e c u r r e n t s t h a t r e s u l t f r o m g e n e r a t i o n o f excess c a r r i e r s by t h e i n c i d e n t beam and s e p a r a t i o n o f t h e s e c a r r i e r s by a p-n j u n c t i o n o r S c h o t t k y b a r r i e r . The EBIC s i g n a l s have been used t o l o c a t e p-n j u n c t i o n s and t o measure t h e d i f f u s i o n l e n g t h o f excess c a r r i e r s . Three geometries have been used f o r d i f f u s i o n l e n g t h measurements namely a ) a p-n j u n c t i o n p e r p e n d i c u l a r t o t h e s u r f a c e , and t h e beam n o r m a l l y i n c i d e n t on t h e s u r f a c e a t v a r i a b l e d i s t a n c e s f r o m t h e j u n c t i o n 7 ; b ) a n g l e l a p p e d s u r f a c e s w i t h t h e p-n j u n c t i o n a t a small a n g l e w i t h r e s p e c t t o t h e s u r f a c e and v a r i a b l e d i s t a n c e s o f t h e e l e c t r o n beam f r o m t h e j u n c - t i o n ; c ) S c h o t t k y b a r r i e r d i o d e p a r a l l e l t o t h e s u r f a c e bombarded w i t h a n o r m a l l y i n c i d e n t e l e c t r o n beam and v a r i a b l e e l e c t r o n beam ~ o l t a g e . ~ F i g u r e 7 shows a com- p a r i s o n o f t h e o r y and experiment f o r case a). F i g u r e 8 shows t h e c a l c u l a t e d r e s u l t s f o r case c ) which i s p a r t i c u l a r l y s u i t e d t o measurement o f s m a l l excess c a r r i e r d i f f u s i o n l e n g t h s .

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6 0 50 40 30 20 10 0 10 DISTANCE (MICRONS)

BEAM V O L T A G E ( K V )

F i g . 7 F i g . 8

F i g . 7 - V a r i a t i o n o f t h e s h o r t c i r c u i t c u r r e n t w i t h probe p o s i t i o n . 7 For DR3 t h e l i n e a r b e h a v i o r a t l a r g e d i s t a n c e s i n t h e n r e g i o n has been s u b t r a c t e d f r o m t h e e x p e r i m e n t a l c u r v e t o o b t a i n t h e s t r a i g h t l i n e o f s t e e p e r s l o p e .

F i g . 8 - T h e o r e t i c a l EBIC c o l l e c t i o n e f f i c i e n c y e f o r Au-GaAs S c h o t t k y b a r r i e r diodes as a f u n c t i o n o f t h e beam v o l t a g e . 8 S o l i d curves were c a l c u l a t e d f o r a g o l d l a y e r t h i c k n e s s o f z, = 250W and a d e p l e t i o n l a y e r w i d t h o f zd = 0.2 p w i t h v a r i o u s d i f f u s i o n l e n g t h s L. Dash and dash-dot curves were c a l c u l a t e d f o r zm = 500a and zm = 15OOA, r e s p e c t i v e l y , w i t h zd = 0.2 u and L = 1.0 p .

Secondary I o n Mass Spectrometry

Secondary i o n mass s p e c t r o m e t r y has been u s e f u l t o determine t h e c o n c e n t r a t i o n o f i m p u r i t i e s a t l o w c o n c e n t r a t i o n s and t o s t u d y t h e d i s t r i b u t i o n o f i m p u n concen- t r a t i o n s w i t h depth. An example i s shown i n F i g . 9 . T h i s f i g u r e showsfdepth p r o f i l e o f s e v e r a l i o n species f o r a specimen o f C r doped GaAs. I n t h i s case i t was i m p o r t a n t t o understand t h e reason f o r t h rmal c o n v e r s i o n o f a s u r f a c e l a y e r t o a l o w r e s i s t i v i t y l a y e r a f t e r h e a t t r e a t m e n t .

5

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5r

I I I I ^I I ^I

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DEPTH (MICRONS)

F i g . 9 - L o g a r i t h m i c p l o t o f i o n i n t e n s i t i e s f o r " ~ r , 2 8 ~ i , 12c, 5 5 ~ n , and 6 3 ~ u vs d e p t h f o r a specimen o f GaAs:Cr h e a t t r e a t e d i n Hz f o r 1 / 2 h a t 800°C.

References

1. WITTRY, D. B., AXELROD, J. M. and McCALDIN, J. O., P r o ~ e r t i e s o f Elemental and Comoound Semiconductors, I n t e r s c i ence Pub1 i shers, NY ( 1 9 6 0 r 8 9 . 2. MARCINIAK, H. C. and WITTRY, D. B., J. App. Phys. 3 (1975) 4823.

3. YIN, S. Y. and WITTRY, D. B., J. Appl . Phys. 54 ( 1 983) 360.

4. LIN, W. N. and WITTRY, D. B., J. Appl. Phys.

5

5. WITTRY, D. B. and KYSER, D. F., J. Appl. Phys. 3 (1967) 375.

6. McCOY, J. H. and WITTRY, D. B., J. Appl . Phys. 42 (1971) 1174.

7. WITTRY, D. B. and Kyser, D. F., J. Appl. Phys. 36 (1965) 1387 8. NU, C. J. and WITTRY, D. B., J. Appl. Phys. 49 (1978) 2827.