PHOTOLUMINESCENCE IN HEAVILY DOPED Si AND Ge

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PHOTOLUMINESCENCE IN HEAVILY DOPED Si AND Ge

J. Wagner, A. Compaan, A. Axmann

To cite this version:

J. Wagner, A. Compaan, A. Axmann. PHOTOLUMINESCENCE IN HEAVILY DOPED Si AND Ge. Journal de Physique Colloques, 1983, 44 (C5), pp.C5-61-C5-64. �10.1051/jphyscol:1983508�. �jpa- 00223088�

JOURNAL DE PHYSIQUE

Colloque C5, suppl6rnent au nO1O, Tome 44, octobre 1983 page C5-61

PHOTOLUMINESCENCE IN HEAVILY DOPED Si AND Ge

J. Wagner, A. cornpaan' and A. Axmann*

Max-PZanck-Institut fiir Festkiirperforschung, Heisenbergstr. 1, 7000 Stuttgart 80, F. R. G.

*Fraunhofer-Institut fur Angewandte FestkBrperphysik, Eckerstr. 4, 7800 Freiburg, F. R. G.

resum6 - iious prksentons des spectres de photoluminescence d basse temp6ra- t u r e mesurks sur une s k r i e d ' k c h a n t i l l o n s de S i e t de Ge dop6s e t implantes en i o n s l o u r d s . En operant un r e c u i t l a s e r sur l e s 6 c h a n t i l l o n s implantks, des concentrations de p o r t e u r s a l l a n t b i e n au-deld de l a l i m i t e de s o l u b i l i - t 6 d l l i q u i l i b r e peuvent E t r e obtenues ( j u s q u ' d l o z 1 ~ m - ~ ) . Oe ces spectres nous dkduisons des i n f o r m a t i o n s sur l a bande i n t e r d i t e r 6 d u i t e a i n s i que sur l e remplissage des bandes en f o n c t i o n de l a c o n c e n t r a t i o n de p o r t e u r s . A b s t r a c t - We r e p o r t t h e r e s u l t s o f low-temperature photoluminescence mea- surements on a s e r i e s o f b u l k doped and h e a v i l y i o n i n p l a n t e d S i and Ge samples. B y l a s e r annealinq of t h e i o n implanted samples c a r r i e r concentra- t i o n s f a r above t h e e q u i l i b r i u m s o l u b i l i t y l i m i t (up t o lo2' ~ m - ~ ) were ob- tained. From t h i s photoluminescence spectra, i n f o r m a t i o n on t h e reduced band gap and t h e band f i l l i n g as a f u n c t i o n of c a r r i e r c o n c e n t r a t i o n i s obtained.

The basic physical p r o p e r t i e s o f h e a v i l y doped semiconductors a r e o f p r a c t i c a l as w e l l as t h e o r e t i c a l i n t e r e s t . For b u l k dooed m a t e r i a l t h e s u b s t i t u t i o n a l i m p u r i t y c o n c e n t r a t i o n i s l i m i t e d t o 10'' - loz0 cm-3 by t h e e q u i l i b r i u m s o l u b i l i t y l i m i t . I t has been shown p r e v i o u s l y t h a t c o n c e n t r a t i o n f a r above t h i s l i m i t can be ob- t a i n e d by l a s e r annealing o f h e a v i l y i o n implanted m a t e r i a l /I/.

We performed photoluminescence s t u d i e s on both b u l k doped and i o n implanted S i and Ge samples w i t h c a r r i e r c o n c e n t r a t i o n up t o loz1 cmT3. By photoluminescence (PL) ex- periments t h e c a r r i e r d i s t r i b u t i o n w i t h i n t h e conduction o r valence band as w e l l as t h e c a r r i e r induced shrinkage o f t h e band gap can be s t u d i e d v i a t h e r a d i a t i v e r e - combination o f photoexcited m i n o r i t y c a r r i e r s /2,3/.

The i o n implanted samples were bombarded w i t h up t o 2

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Fig. 1 shows t h e r e s u l t s f o r b u l k doped Si:P. I n t h e PL spectrum o f t h e sample w i t h t h e l o w e s t i m p u r i t y c a n c e n t r a t i o n ( 4

.

These are, from higher t o lower energies, the n o - p h o n o n - t r a n s i t i o n p ) , t h e TA and TO momentum conserving phonon a s s i s t e a r e p l i c a s and t h e TO + 0 r e p l i c a , i n - v o l v i n g a momentum conserving p l u s an o p t i c a l zone center phonon. For i n c r e a s i n g i m p u r i t y c o n c e n t r a t i o n t h e l i n e s become broader and f o r t h e heaviest doped samples o n l y the two s t r o n g e s t rep1 i c a s (NP and TO) can be d i s t i n g u i s h e d . From t h e e x c i t a - t i o n d e n s i t y o f = l o 0 W/cm z used f o r r e c o r d i n g t h i s spectra i t can b e concluded t h a t

t Permanent address:

Department of Physics, Cardwell Hall, Kansas State University, Manhattan, Kansas 66506, U.S.A.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983508

0 - 6 2 JOURNAL DE PHYSIQUE

we a r e i n t h e h i g h e x c i t a t i o n l i m i t as d e f i n e d by Parsons 151. Therefore the l u m i - nescence i s i n t e r p r e t e d as t h e r a d i a t i v e recombination o f e l e c t r o n s from t h e dege- nerate conduction band w i t h f r e e photoexcited holes.

Photon Energy feV) 0.9

1.4 1.3 1.2 1.1 1.0

Wavelength (ym)

Photon Energy (eV)

1.7 1.6 1.5 1.4 1.3 1.2 Wavelength (pm)

Fiq. I

-

arrows i n d i c a t e t h e h i g h energy ( 4

.

-

c u t o f f edge o f t h e [UP-line (EGal) cm-2 and 2 1016 i m p l a n t a t i o n dose)

.

and t h e low energy edge o f t h e Ge:P samples. The i m p l a n t a t i o n o f 1

.

TO-rep1 i c a (EG,2 - haT0).

cm-2 corresponds t o about 2-3

.

t h e one o f 2

-

-

~ r n - ~ c a r r i e r concentration. The arrows i n d i -

c a t e t h e h i g h energy edge EG-l o f t h e lumines-

-. ^> -

cence band. The drawn l i n e s a r e spectra n o t c o r r e c t e d f o r d e t e c t o r response. The dashed l i n e s i n d i c a t e t h e luginescence l i n e shape a f t e r a p p l y i n g t h i s c o r r e c t i o n .

The low energy c u t - o f f o f the luminescence l i n e , i n d i c a t i n g t h e energy separation EG,2 a t t h e top of t h e valence and bottom o f t h e conduction band, e x h i b i t s a pro- nounced s h i f t t o lower energies w i t h i n c r e a s i n g c a r r i e r c o n c e n t r a t i o n . The h i g h energy edge EG,l, given by t h e sum o f t h e band gap EG,2 and t h e k i n e t i c energy of

t h e c a r r i e r s , remains approximately constant w i t h i n c r e a s i n g i m p u r i t y concentration.

Fig. 3 - Bandstructure o f Ge. The luminescence t r a n s i t i o n s , p o s s i b l e i n h e a v i l y doped n-type m a t e r i a l , a r e schematically i n d i c a t e d . The values shown f o r band sepa- r a t i o n s r e f e r t o pure Ge a t low temperatures.

The r e s u l t s f o r Ge:P a r e summarized i n F i g . 2. The sample w i t h t h e lowest i m p u r i t y c o n c e n t r a t i o n i s b u l k doped, t h e more h e a v i l y doped ones a r e i o n implanted. The l u - minescence spectrum, here again o r i g i n a t i n g from band t o band t r a n s i t i o n s , i s domi- nated by t h e no-phonon c o n t r i b u t i o n 121. The h i g h energy edge shows a d r a s t i c s h i f t t o higher energies w i t h i n c r e a s i n g c a r r i e r concentration. For t h e heaviest i m p l a n t - ed sample (25

-

.

values r e f e r t o pure Ge a t low temperatures. The low energy edge o f t h e PI ^{1 in e}

c o u l d n o t be r e s o l v e d due t o t h e cut-off a t 1.7 um o f t h e Ge d e t e c t o r used f o r these experiments.

Only a r e l a t i v e l y small number o f e l e c t r o n s c o n t r i b u t e t o t h e r ^{t o} r luminescence due t o t h e much lower d e n s i t y o f s t a t e s i n the r c band minimum 8ompar8d t o t h e 4 - f o l d degenerated L p o i n t minima. But on the o t h e r hand t h e luminescence e f f i c i e n c y i s expected t o be much h i g h e r f o r t h i s d i r e c t t r a n s i t i o n than f o r i n d i r e c t proces- ses. So t h e luminescence i n t e n s i t y of t h e d i r e c t t r a n s i t i o n becomes comparable t o t h e one o f t h e i n d i r e c t L t o r v recombination. D i r e c t luminescence from the r c con-

C

d u c t i o n band minimum has been r e p o r t e d p r e v i o u s l y f o r h i g h l y photoexcited pure Ge by van D r i e l e t a l . 161. The low temperature PL spectra shown i n t h a t paper have s t r o n g s i m i l a r i t i e s t o t h e one shown i n t h e present work f o r t h e most h e a v i l y doped sample.

Comparing t h e dependence o f t h e o p t i c a l gap EGS1 on t h e c a r r i e r c o n c e n t r a t i o n f o r n-type Ge and S i a remarkable d i f f e r e n c e i s found. I n Ge a pronounced s h i f t t o higher energies i s observed f o r i n c r e a s i n g dopant c o n c e n t r a t i o n s . T h i s s h i f t amounts t o ~ 0 . 2 5 eV f o r 5

.

JOURNAL DE PHYSIQUE

t r a s t an a p p r o x i m a t e l y c o n s t a n t v a l u e o f EG-l i s f o u n d u p t o 1.5 lo2' CVI-~. As p o i n t e d o u t by Mahan / 7 / t h i s d i f f e r e n t b e h a v i o u r i s a t t r i b u t e d t o two e f f e c t s . F i r s t t h e e f f e c t i v e d e n s i t y o f s t a t e s mass f o r e l e c t r o n s i s a f a c t o r o f two s m a l l e r f o r Ge t h a n f o r S i when t a k i n g i n t o account t h e degeneracy o f t h e c o n d u c t i o n band.

T h i s e f f e c t r e s u l t s i n a h i g h e r k i n e t i c energy i n Ge t h a n i n S i f o r a g i v e n c a r r i e r c o n c e n t r a t i o n . Second t h e r e d u c t i o n o f t h e gap EG,p i s l a r g e r i n S i t h a n i n Ge due t o t h e l a r g e r Rydberg energy o f S i .

I n c o n c l u s i o n we have shown t h a t semiconductors e x t r e m e l y h e a v i l y doped b y i o n im- p l a n t a t i o n and l a s e r a n n e a l i n g can be s t u d i e d by photoluminescence. T h i s s t u d y p r o v i d e s v a l u a b l e i n f o r m a t i o n on t h e f i l l i n g o f h i g h e r l y i n g band minima and on t h e band gap s h i f t as a f u n c t i o n o f c a r r i e r c o n c e n t r a t i o n o v e r a much w i d e r range as a c c e s s i b l e w i t h b u l k doped m a t e r i a l .

ACKNOWLEDGEMENTS

We l i k e t o t h a n k P r o f . M. Cardona f o r many h e l p f u l d i s c u s s i o n s and Messrs. H. H i r t ,

PI. Siemers and P. Wurster f o r v a l u a b l e e x p e r i m e n t a l a s s i s t a n c e .

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