THE E1 - E1 + Δ1 TRANSITIONS IN BULK GROWN AND IN IMPLANTED LASER ANNEALED HEAVILY DOPED GERMANIUM : LUMINESCENCE

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THE E1 - E1 + ∆1 TRANSITIONS IN BULK GROWN AND IN IMPLANTED LASER ANNEALED HEAVILY

DOPED GERMANIUM : LUMINESCENCE

G. Contreras, A. Compaan, J. Wagner, M. Cardona, A. Axmann

To cite this version:

G. Contreras, A. Compaan, J. Wagner, M. Cardona, A. Axmann. THE E1 - E1 + ∆1 TRAN- SITIONS IN BULK GROWN AND IN IMPLANTED LASER ANNEALED HEAVILY DOPED GERMANIUM : LUMINESCENCE. Journal de Physique Colloques, 1983, 44 (C5), pp.C5-55-C5-59.

�10.1051/jphyscol:1983507�. �jpa-00223087�

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J O U R N A L D E PHYSIQUE

Colloque C5, suppl6ment au nO1O, _Tome44, octobre 1983 page C5-55

THE El - E l + A1 TRANSITIONS I N BULK GROWN AND I N IMPLANTED LASER ANNEALED H E A V I L Y DOPED GERMANIUM : LUMINESCENCE

G. ~ontreras+, A. Compaan ++ , J. Wagner, M. Cardona and A. ~ x m a n n *

Max-PZanck-Institut fiir FestkGrperforschung, Heisenbergstr. I , 7000 Stuttgart 80, F.R.G.

'Fraunhofer-Institut fiir Angewandte FestkBrperphysik, Eekerstr. 4, 7800 Freiburg, F. R. G.

R'esum'e - Nous pr'esentons des mesures de luminescence v i s i b l e e x c i t ' e e p a r un l a s e r c o n t i n u e t l i ' e e aux t r a n s i t i o n s d i r e c t e s El, El + Al dans Ge t y p e n avec N 1018 Le phPnomPne semble P t r e i n d u i t p a r l a p a r t i c i p a t i o n d ' u n e hgute d e n s i t 6 d 1 P 7 e c t r o n s l i b r e s aux t r a n s i t i o n s El - El + ^A a y a n t l i e u aux

1

e n v i r o n s du p o i n t L (2,l e t 2,3 eV): c e t t e luminescence n ' e s t observ6e n i dans 1 e Ge pur, n i dans Ge t y p e p . C e t t e luminescence e s t u t i l i s P e pour 1 'Ptude de l ' i n f l u e n c e du dopage s u r l a s t r u c t u r e de bandes du Ge dop'e en volume e t a u s s i i m p l a n t s avec du P e t r e c u i t p a r l a s e r . Des d'eplacements e t ' e l a r g i s s e - ments des 'energies El - El + al o n t P t P observ6s a i n s i que des d'eplacements Burstein-Moss de 1 ' P n e r g i e d e ~ e r m i . Les r @ s u l t a t s s u r l e Ge dop'e en volume s o n t en bon a c c o r d avec ceux du m a t 6 r i a u r e c u i t p a r l a s e r dans l a r'egion od l e dopage e s t l e m0me.

A b s t r a c t - We r e p o r t t h e o b s e r v a t i o n o f cw l a s e r - e x c i t e d v i s i b l e l u m i n e s - - - cence a t t h e El

-

El + Al d i r e c t gaps i n n - t y p e Ge w i t h Ne

"

10'' c m - 5 The phenomenon appears t o be induced by p a r t i c i p a t i o n o f t h e h i g h d e n s i t y o f f r e e e l e c t r o n s i n t h e El - El + al t r a n s i t i o n s a t t h e L - p o i n t (2.1 and 2.3 eV) s i n c e t h e luminescence i s n o t observed i n e i t h e r p u r e o r p - t y p e Ge. We have used t h i s luminescence t o i n v e s t i g a t e t h e e f f e c t o f doping on t h e band s t r u c t u r e o f b u l k doped and a l s o o f phosphorus i m p l a n t e d l a s e r annealed Ge. S h i f t s and broadening o f t h e El - E + al e n e r g i e s and a l s o B u r s t e i n - Moss-Fermi energy s h i f t s have been obsehved. The r e s u l t s f o r b u l k Ge go o v e r smoothly and o v e r l a p w i t h t h e r e s u l t s f o r t h e l a s e r annealed m a t e r i a l .

The e f f e c t s o f heavy d o p i n g o n t h e e l e c t r o n i c band s t r u c t u r e o f semiconductors have been v e r y p r o d u c t i v e l y s t u d i e d v i a t h e l o w e s t band-to-band (edge) r e c o m b i n a t i o n l u - minescence s p e c t r a /I/. I n t h e case o f S i and Ge t h i s luminescence, a t t h e i n d i r e c t gaps i n t h e near i n f r a r e d , has p r o v i d e d i n f o r m a t i o n on t h e c a r r i e r - i n d u c e d r e n o r - m a l i z a t i o n o f t h e band edges, and t h e i n c r e a s e o f t h e c a r r i e r Fermi l e v e l (band f i l l i n g ) w i t h i n c r e a s i n g c o n c e n t r a t i o n . The h i g h e r l y i n g d i r e c t gaps i n S i and Ge have, however, been l e s s t r a c t a b l e f o r such s t u d i e s s i n c e t h e y do n o t n o r m a l l y show s i g n i f i c a n t r e c o m b i n a t i o n luminescence. Thus such s t u d i e s had t o be performed u s i n g r e f l e c t a n c e o r e l l i p s o m e t r i c t e c h n i q u e s / 2 / . We r e p o r t here t h e o b s e r v a t i o n of s t r o n g r e c o m b i n a t i o n luminescence near 2.2 eV i n Ge, f a r above t h e i n d i r e c t edge luminescence ( ~ 0 . 7 8 eV). We s h a l l p r e s e n t evidence t h a t t h i s v i s i b l e luminescence

'Permanent address :

++ DAAD Fellow, on leave from E.S.F.M.-I.P.N. Me'xico.

Permanent address :

Von Humboldt Foundation Fellow, on leave from Kansas State University, Dept. of Physics, Cardwell Hall, Manhattan, Kansas 66506, U.S.A.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983507

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C5-56 JOURNAL DE PHYSIQUE

a r i s e s from d i r e c t e l e c t r o n - h o l e bimolecular recombination a t t h e E, ₁ - El ₁ + ^A,₁^gap

a t t h e L-point of t h e B r i l l o u i n zone. The recombination r a t e appears t o be enhanced by t h e presence of a high d e n s i t y of e l e c t r o n s a t t h e L1 conduction band v a l l e y s . The samples used i n t h i s study included several bulk samples doped with P o r As and f i v e samples implanted with P a t 190 keV with doses of 2 , 5, 10, 20, and

60 x 1015 The implanted samples were r e p e t i t i v e l y pulsed-laser-annealed with

,,-

a XeCl l a s e r using an energy d e n s i t y of 20.83/cmL a s described i n more d e t a i l i n Ref. 3. The p o s i t i o n of t h e i n f r a r e d r e f l e c t i v i t y minimum i n d i c a t e s f r e e c a r r i e r c o n c e n t r a t i o n s of 6 x 10'' and 1 x 10'' f o r t h e two most l i g h t l y implanted samples a f t e r 1 a s e r annealing. For t h e t h r e e heaviest implants t h e i n f r a r e d r e f l e c - t i v i t y s p e c t r a imply s t r o n g d e v i a t i o n s from s i n g l e e f f e c t i v e mass behavior of t h e e l e c t r o n s . We b e l i e v e t h i s a r i s e s from f i l l i n g of t h e r and X-point v a l l e y s a t t h e higher d e n s i t i e s , a s i n d i c a t e d i n t h e photoluminescence r e s u l t s of Wagner e t a1./4/

( s e e below). From p a r a l l e l Raman s t u d i e s of t h e i n t e n s i t y of t h e phosphorous local mode /5/ i n t h e t h r e e most heavily doped samples, we b e l i e v e t h a t t h e P i s predo- minantly s u b s t i t u t i o n a l . Hence we e s t i m a t e t h e e l e c t r o n d e n s i t i e s i n t h e s e t h r e e samples t o be %2 x loz0, ^{4 x} and -1 x lo2' ~ m - ~ .

PHOTON ENERGY ( e V 1

Fig. 1 - Room temperature photoluminescence from P implanted pulsed l a s e r annealed Ge f o r e x c i t a t i o n with t h r e e d i f f e r e n t Krf l a s e r l i n e s .

Figure 1 shows t h e photoluminescence s p e c t r a obtained with approximately 100 mW of cw r a d i a t i o n from several Krf l a s e r l i n e s . The d a t a , p l o t t e d on an a b s o l u t e energy s c a l e , c l e a r l y show t h e c h a r a c t e r i s t i c s of photoluminescence r a t h e r than an i n e l a -

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s t i c s c a t t e r i n g process which should s h i f t with t h e l a s e r l i n e . These data were taken a t room temperature. S i m i l a r s p e c t r a were observed from As bulk doped and implanted samples but not from pure Ge o r from Ga o r B implanted (p-type) Ge samples.

F i g . 2 - Temperature dependence of photoluminescence observed i n bulk doped Ge:P ( 2 1 x 1019 ~ m - ~ ) . E x c i t a t i o n photon energy was 2.71 eV.

- - 2

3 Z

F z

0

z

W 0 m

W

z z 2

The s p e c t r a of Figure 2 show c l e a r l y t h e s h i f t with temperature and t h e narrowing of t h e two peaks. The s h i f t of 80 meV agrees reasonably with t h e temperature s h i f t o f 90 meV measured i n o p t i c a l s p e c t r a / 2 / .

2.22 ev 2.1 x 1019

4

2 . 4 2 eV

- 7 7 K

z

^2.14

⁴

^eV _{2.34 eV}

293K

Fig. 3 - Photoluminescence a t 77 K f o r several P dopant concentrations and f o r pure Ge. Excitation a t hv = 2.71 eV. Narrow peak t o t h e r i g h t i s t h e 300 cm-I Raman l i n e of Ge.

1.9

, >

^2.6

PHOTON ENERGY (eV )

PHOTON ENERGY (eV)

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C5-58 JOURNAL DE PHYSIQUE

F i g u r e 3 shows t h e dependence o f t h i s luminescence on c a r r i e r d e n s i t y f o r d e n s i t i e s

- -

r a n g i n g ' f r o m 1.6 x 10'' t o -1 x 10" a t l o w temperatures. A t t h e l o w e r d e n s i t i e s (bulk-doped samples) one c l e a r l y observes d o u b l e peaks separated by t h e s p i n - o r b i t s p l i t t i n g o f t h e v a l e n c e band a t t h e L - p o i n t (0,20 eV). The p o s i t i o n o f t h e s e l u - minescence peaks agrees w e l l w i t h observed s t r u c t u r e s i n e l l i p s o m e t r i c s p e c t r a w h i c h have been i d e n t i f i e d w i t h d i r e c t band-to-band t r a n s i t i o n s a t t h e L - p o i n t and a l o n g t h e A l i n e s (El and El +

9

^gaps)/ 6 / . The spectrum f r o m t h e sample i m p l a n t e d w i t h 2 x 1015 ~ + / c m ( n o t shown) d i s p l a y s peaks which a r e s i m i l a r t o b u t somewhat 2 b r o a d e r t h a n t h o s e o f t h e b u l k sample w i t h 2 . 1 x 1019 p/cm2. T h i s r e s u l t i s i n qua- l i t a t i v e agreement w i t h t h e i n f r a r e d r e f l e c t i v i t y w h i c h i m p l i e s a c a r r i e r c o n c e n t r a - t i o n of 6 x 10'' f o r t h i s i m p l a n t e d sample. Hence d o p i n g by i m p l a n t a t i o n and b u l k d o p i n g a r e seen t o be e q u i v a l e n t w i t h r e g a r d s t o t h e El - El t Al luminescence d i s c u s s e d here. We have observed weak luminescence f r o m n - t h e - . G; s a m i l e s w i t h con- c e n t r a t i o n s as l o w as 8 x 1018 ~ m - ~ . Thus t h e luminescence s p e c t r a develop i n a mo- n o t o n i c way f r o m t h e b u l k samples t o t h e h e a v i e s t i m p l a n t e d sample.

The photoluminescence i n t e n s i t y from m a t e r i a l s w i t h v e r y s m a l l a b s o r p t i o n l e n g t h s f o r l i g h t , such as t h e case h e r e (cl 2. lom6 cm), i s v e r y s e n s i t i v e t o s u r f a c e con- d i t i o n s . However, w i t h i n t h e l i m i t s imposed b y s u r f a c e v a r i a b i l i t y f r o m sample-to- sample, t h e t o t a l El, El + Al photoluminescence appears t o s c a l e l i n e a r l y w i t h t h e c a r r i e r c o n c e n t r a t i o n . The one s i g n i f i c a n t e x c e p t i o n i s t h e h e a v i e s t i m p l a n t e d sample f o r w h i c h t h e i n t e g r a t e d s i g n a l from 1.8 eV t o 2.6 eV i s s l i q h t l y l e s s t h a n t h a t o f t h e second h i g h e s t i m p l a n t ( 2 x 1016 P/cm 2 ) . T h i s may a r i s e f r o m i n c o m p l e t e P s u b s t i t u t i o n a l i t y , P-P p a i r i n g , o r from s i g n i f i c a n t luminescence below 1.8 eV.

A more l i k e l y s o u r c e o f t h i s decrease i s t h e f i l l i n g o f h i g h c o n d u c t i o n band minima such as t h o s e a t r a n d a t X / 4 / .

The E - El + A l photoluminescence d i s c u s s e d above i s c h a r a c t e r i z e d by t h e f o l l o w - i n g f a c t s :

I t r e q u i r e s t h e presence o f a l a r g e d e n s i t y c 1 0 1 9 cmm3) o f e l e c t r o n s i n t h e L v a l l e y s and i s r o u g h l y p r o p o r t i o n a l t o t h i s number. I t i s n o t observed f o r h e a d i l y doped p - t y p e samples. P-type d o p i n g does n o t i n d u c e t h i s luminescence. An equiva- l e n t luminescence i s n o t observed a t t h e El - El + A gaps o f h e a v i l y doped ( e i t h e r ~ n o r p ) S i (2.3.4 eV).

Because o f t h e s i m i l a r i t y of t h e observed luminescence s t r u c t u r e and t h e correspond- i n g s t r u c t u r e i n t h e o p t i c a l c o n s t a n t s (El, El +al, see, e.g., F i g s . 121 and 122 o f Ref. 2) i t i s c l e a r t h a t t h e luminescence corresponds t o n e a r l y d i r e c t recombi- n a t i o n o f e l e c t r o n s n e a r t h e s t a t e s w i t h A3 h o l e s . I n d i r e c t , k-n?ifiEEerving t r a n s i t i o n s a r e l a r g e l y excluded as t h e y would m o s t l y b l u r t h e El - nl + al s t r u c - t u r e i n a manner s i m i l a r t o t h a t o f amorphous Ge. The f a c t t h a t t h e luminescence i n - t e n s i t y i s p r o p o r t i o n a l t o t h e c o n c e n t r a t i o n Ne o f e l e c t r o n s a t t h e L1 minima ( h o l e s a t have no i n f l u e n c e , n o r e l e c t r o n s a t al such as i n S i ) i n d i c a t e s t h a t t h e l u m i n e s c e n t t r a n s i t i o n s must i n v o l v e e l e c t r o n s near L1 and h o l e s near L 3 , , i . e . , o n l y a s m a l l f r a c t i o n o f t h e A3 ^-tAl t r a n s i t i o n s which c o n t r i b u t e t o t h e s t r u c t u r e i n a b s o r p t i o n . The p r o p o r t i o n a l i t y t o t h e i n c i d e n t i n t e n s i t y t o Ne a l s o suggests

" b i m o l e c u l a r " r e c o m b i n a t i o n , i .e., t h e luminescence i s p r o p o r t i o n a l t o t h e p r o d u c t (Ne + ne)pe, where ne = pe a r e t h e d e n s i t i e s of p h o t o e x c i t e d e l e c t r o n s and h o l e s near L3, and N, z> np. The " b i m o l e c u l a r " process excludes e x a c t d i r e c t t r a n s i t i o n s ( p r o -

p o r t i o n a l - o n l y t o p ) . Hence a r a n g e AX o f non c o n s e r v a t i o n o f k, such t h a t AX <.IF,

t h e Fermi momentum %f t h e f r e e e l e c t r o n s , i s r e q u i r e d t o e x p l a i n t h e r e s u l t s . T h ~ s n o n - c o n s e r v a t i o n o f k i s l i k e l y t o a r i s e f r o m s c a t t e r i n g b y t h e screened donor i o n s .

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The s t r e n g t h o f t h e luminescence r e q u i r e s t h a t e x c i t e d holes s t a y a s u f f i c i e n t l y l o n g t i m e near t h e L3, s t a t e s i n s p i t e o f t h e f a c t t h a t these s t a t e s are ~ 1 . 3 eV below t h e t o p o f t h e valence band. A l o n g t h e r r n a l i z a t i o n time f o r these s t a t e s may a r i s e from t h e anomalously small values o f t h e i n t r a b a n d s l e c t r o n o p t i c a l phonon c o u p l i n g constant a t t h e L3, s t a t e s / 7 / .

Attempts t o f i t t h e luminescence curves w i t h f u n c t i o n s which r e f l e c t t h e d e n s i t y o f occupied s t a t e s near L1 and L , have so f a r f a i l e d . The observed lineshapes a r e b a s i c a l l y Lorentzian, any d e i s i t y o f s t a t e s s t r u c t u r e being probably broadened by l i f e t i m e e f f e c t s . The L o r e n t z i a n widths increase w i t h i n c r e a s i n g Ne. For 2 x 10'' ~ m - ~ , f o r instance, we f i n d h,-l = 0.1 eV.

ACKNOWLEDGEMENTS

We are g r a t e f u l t o T.P. M a r t i n f o r use o f t h e excimer l a s e r and t o H. Breitschwerdt f o r measuring t h e i n f r a r e d r e f l e c t i o n spectra. Thanks a r e a l s o due t o G. K i s e l a f o r sample preparation and t o M. Siemers, P. Wurster, and H. H i r t f o r technical assistance

REFERENCES

1. See, e.g., OLEGO D. and CARDONA M., Phys. Rev. B 22 (1980) 886;

BENOIT A LA GUILLAUME C. and CENROGORA J., Phys. =at. Sol. 35 (1968) 599;

SCHMID P.E., THEWALD M.L.W., and DUMKE W.P., S o l i d S t a t e CornEn.

38

(1981) 1091.

2. Landoldt-Bornstein Tables, Vol

.

17a, ed. by 0. Madelung, M. Schulz, and H. Weiss (Springer Verlag, B e r l i n , 1982), p. 90 and references t h e r e i n . 3. COMPAAN A., CONTRERAS G., CARDONA M. and AXMANN A., proceedings o f t h i s

conference.

4. WAGNER J., COMPAAN A. and AXMANN A., proceedings o f t h i s conference.

5. CONTRERAS G., COMPAAN A. and AXMANN A., proceedings o f t h i s conference.

6. VINA L. and CARDONA M., i n "Physics o f Semiconductors1', ed. by M. Averous (North-Holland, Amsterdam, 1982), p. 356, a l s o unpublished data f o r implanted m a t e r i a l .

7. RENUCCI M.A., RENUCCI J.B., ZEYHER R. and CARDONA M., Phys. Rev. B 10, (1974)

42ns