INTERACTION OF ULTRASONIC PHONONS WITH DONOR IMPURITIES IN HYDROGENATED AMORPHOUS GERMANIUM

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INTERACTION OF ULTRASONIC PHONONS WITH DONOR IMPURITIES IN HYDROGENATED

AMORPHOUS GERMANIUM

K. Bhatia, S. Hunklinger

To cite this version:

K. Bhatia, S. Hunklinger. INTERACTION OF ULTRASONIC PHONONS WITH DONOR IMPU-

RITIES IN HYDROGENATED AMORPHOUS GERMANIUM. Journal de Physique Colloques, 1981,

42 (C4), pp.C4-479-C4-482. �10.1051/jphyscol:19814101�. �jpa-00220716�

CoZZoque C4, suppZ6ment au nO1O, Tome 42, octobre 1981

INTERACTION OF ULTRASONIC PHONONS WITH DONOR IMPURITIES IN HYDROGENATED AMORPHOUS GERMANIUM

K.L. ~ h a t i a " and S. ~ u n k l i n g e i

MaarPZanck-Institut fur Festkdrperforschung, Heisenbergstrasse 1, 0-7000 Stuttgart 80, F. R. G.

" ^On leave from

:

Department of Physics, Maharshi Dayanand University, Rohtak-124001, India

A b s t r a c t . - We have s t u d i e d t h e propagation of a c o u s t i c s u r f a c e waves a t tem- p e r a t u r e s between 0.45 K and 475 K i n hydrogenated amorphous germanium doped with phosphorous. A t low c o n c e n t r a t i o n s a s t r o n g a b s o r p t i o n maximum a t 18 K i s observed, whereas a t high doping r a t e s t h i s a b s o r p t i o n i s suppressed. I n s i m i l a r i t y t o c r y s t a l s our r e s u l t s can b e explained by t h e i n t e r a c t i o n between t h e sound wave and t h e d e g e n e r a t e donor ground s t a t e .

I n t r o d u c t i o n . - A c u r r e n t focus of i n t e r e s t i n t h e f i e l d of amorphous semiconductors i s t h e q u e s t i o n of s u b s t i t u t i o n a l doping. The demonstration / 1 , 2 / t h a t amorphous s i l i c o n and germanium can be doped t o e x h i b i t n-type o r p-type behaviour, h a s stimu- l a t e d a v a r i e t y of experimental and t h e o r e t i c a l i n v e s t i g a t i o n s / 3 / . I n s p i t e of ex- t e n s i v e experimental s t u d i e s , t h e n a t u r e of donor o r a c c e p t o r i m p u r i t y s t a t e s i n doped a-Si:H and a-Ge:H remains f a r from b e i n g understood /3/. Study of t h e s e impu- r i t y s t a t e s by u l t r a s o n i c methods can provide i n f o r m a t i o n a b o u t t h e i r dynamical pro- p e r t i e s . I n t h i s communication we r e p o r t on experiments i n phosphorous doped a-Ge:H by a c o u s t i c s u r f a c e wave technique.

Experimental.- A s s u b s t r a t e s , we used s t a n d a r d yz-cut LiNb03 s u r f a c e wave d e v i c e s w i t h i n t e r d i g i t a l t r a n s m i t t i n g and r e c e i v i n g t r a n s d u c e r s 10 mm a p a r t . Thin f i l m s ( t h i c k n e s s a b o u t 0.3 vm) of doped hydrogenated a-Ge were r e a c t i v e l y s p u t t e r e d o n t o h e a t e d s u b s t r a t e s ( s u b s t r a t e temperature was about 90 OC) i n a mixture of A r

+

pH3.

Whereas t h e p r e s u r e of hydrogen and t o t a l p r e s s u r e o f argon was k e p t c o n s t a n t a t I X I O - ~ and 5 x 1 6 ' T o r r , r e s p e c t i v e l y , t h e p a r t i a l p r e s s u r e of t h e mixture A r

+

pH3 (1.12% volume pH3 i n A r ) was v a r i e d t o p r e p a r e f i l m s o f d i f f e r e n t phosphorous con- c e n t r a t i o n . No c r y s t a l l i n e r e g i o n i n t h e f i l m could b e d e t e c t e d by X-ray d i f f r a c - t i o n . We e s t i m a t e 3000, 20 and 10 ppm pH3 i n A r i n t h e s p u t t e r i n g plasma. I t i s known t h a t i n t h e doping regime approximately 20% of t h e i m p u r i t y atoms e n t e r /11/

s u b s t i t u t i o n a l l y i n t h e amorphous s i l i c o n network and r e s t a s self-compensated s i t e s . A r e f e r e n c e s u r f a c e wave p a t h on t h e same s u b s t r a t e was l e f t w i t h o u t a f i l m i n o r d e r t o determine and t o s u b t r a c t t h e c o n t r i b u t i o n of t h e LiNb03-substrate. Measurements of t h e a t t e n u a t i o n w i t h temperature were c a r r i e d o u t between 0.45 K and 475 K a t a frequency of 300 MHZ.

The a t t e n u a t i o n aF of an a c o u s t i c s u r f a c e wave p r o p a g a t i n g on a s u b s t r a t e which i s covered by a t h i n f i l m , i s r e l a t e d t o t h e a t t e n u a t i o n ag of a s u r f a c e wave on a bulk sample o f t h e f i l m m a t e r i a l by

+

⁼ ClgoIkh. I i s a c o n s t a n t and depends only on t h e e l a s t i c p r o p e r t i e s of t h e s u b s t r a t e and t h e f i l m , k i s t h e wave v e c t o r and h t h e f i l m t h i c k n e s s . This r e l a t i o n i s v a l i d i f t h e p e n e t r a t i o n depth o f t h e s u r f a c e wave i s much l a r g e r t h a n t h e f i l m t h i c k n e s s , i . e . i f hk < < l /4/. From o u r measured a b s o r p t i o n we s u b t r a c t t h e " r e s i d u a l " a t t e n u a t i o n e s t i m a t e d from t h e l e v e l - i n g o f f of t h e a b s o r p t i o n a t t h e lowest temperatures. I n a l l o u r p l o t s we show t h e v a l u e CIF/Ikh, i . e . t h e a t t e n u a t i o n e s t i m a t e d f o r a s u r f a c e wave on t h e b u l k m a t e r i a l . R e s u l t s and Discussion.- I n F i g u r e ( l ) , the normalized u l t r a s o n i c a b s o r p t i o n of a-Ge(H,P) f l l m s of d i f f e r e n t P c o n c e n t r a t i o n s i s shown a s a f u n c t i o n of temperature.

A t t e n u a t i o n of 9 GHz u l t r a s o n i c waves i n c r y s t a l l i n e Ge(PI t a k e n from r e f e r e n c e / 6 /

*

On l e a v e from : Department of P h y s i c s , Maharshi Dayanand U n i v e r s i t y , Rohtak-124001, I n d i a .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814101

JOURNAL DE PHYSIQUE

F i g . 1:

'OS!

A ---

c-GelP) 9GHz

U l t r a s o n i c a b s o r p t i o n v e r s u s temperature i n a-Ge (H,P) f i l m s

1 0 '

( A ) (--) c-Ge(P) with 1015 i m p u r i t i e s cm-3

I ' ' """I ' ' """I ' ' " '

- B

l a-Ge(H) 300MHz

- ^C

^{o a-Ge(H,P) - , a -}

-

T

D

a-Ge(H,P)

-

¹¹

-

( B ) ( 0 ) a-Ge(H) w i t h 1 9 a t % H which i s a b o u t t h e same a s i n

a-Ge(H,P)

( C ) ^{( 0 )} a-Ge(H,P)

,

3000 ppm PH3 i n Argon

- %

( D ) (I) a - G ~ ( H , P )

,

20 ppm pH3 i n Argon

:

E

A a-Ge(H,P)

-

^{$ 1 -}

- :

(E) ( A ) a-Ge(H,P)

,

10 ppm pH3 i n

Argon

i s a l s o shown i n F i g u r e ( 1 ) ( c u r v e A ) . For comparison t h e a t t e n u a t i o n i n a-Ge(H) f i l m , c o n t a i n i n g approximately t h e same amount of hydrogen a s i n t h e doped f i l m / 5 / , i s p l o t t e d . The a c o u s t i c a b s o r p t i o n i n a-Ge(H,P) shows s i g n i f i c a n t d i f f e r e n c e s from t h a t i n a-Ge(H). For t h e sample E having t h e lowest phosphorous c o n c e n t r a t i o n

(10-ppm) a s t r o n g and broad a b s o r p t i o n peak i s d e t e c t e d a t 18 K which d e c r e a s e s only s l i g h t l y with temperature above 20 K. The magnitude of t h e a b s o r p t i o n i s con- s i d e r a b l y enhanced i f t h e c o n c e n t r a t i o n o f phosphorous i s i n c r e a s e d t o 20 ppm

(sample D ) . A f u r t h e r i n c r e a s e t o 3000 ppm (sample C ) , however, s u p p r e s s e s t h i s s t r o n g a b s o r p t i o n . I n s t e a d , t h e a b s o r p t i o n r i s e s l i n e a r l y with t e m p e r a t u r e upto 30 K a s i n a-Ge(H) / 5 / . Above 30 K a s t e e p r i s e of t h e a b s o r p t i o n s i m i l a r t o t h a t i n a-Ge(H) i s observed, b u t i t s o n s e t i s a t a much lower temperature.

g ¹⁰³ -

I n s e e k i n g an i n t e r p r e t a t i o n i t i s worthwhile t o d i s c u s s f i r s t t h e a t t e n u a - t i o n i n c r y s t a l l i n e germanium doped w i t h phosphorous. I n c-Ge(P) a l s o a maximum i n t h e u l t r a s o n i c a b s o r p t i o n i s observed around 20 K . I t h a s been e x p l a i n e d by t h e s t r o n g s c a t t e r i n g of u l t r a s o n i c phonon by e l e c t r o n s bound t o t h e donor i m p u r i t i e s 6 I n c r y s t a l l i n e germanium t h e donor ground s t a t e i s f o u r - f o l d d e g e n e r a t e due t o t h e m u l t i v a l l e y n a t u r e o f i t s conduction hapd. Because of t h e l o c a l f i e l d t h i s de- generacy i s l i f t e d and t h e ground s t a t e s p l i t s i n t o a s i n g l e t and a t r i p l e t s t a t e s e p a r a t e d by 2.8 meV. An u l t r a s o n i c wave modulates t h e energy s e p a r a t i o n between t h e s i n g l e t and t r i p l e t ground s t a t e s and t h u s d i s t u r b s thermal e q u i l i b r i u m . The i m p u r i t y e l e c t r o n s t r y t o r e e s t a b l i s h e q u i l i b r i u m v i a t r a n s i t i o n between t h e s e s t a t e s by emission o r a b s o r p t i o n o f a thermal phonon. A s u s u a l , such a p r o c e s s l e a d s t o a b s o r p t i o n of u l t r a s o n i c wave. I n amorphous semiconductors t h e wave v e c t o r ke of t h e e l e c t r o n s does n o t remain a good quantum number because o f t h e absence of l o n g range o r d e r . The wave v e c t o r may become u n c e r t a i n w i t h i n a c e r t a i n s p r e a d he.

There i s c l o s e s i m i l a r i t y of t h e r a d i a l d i s t r i b u t i o n f u n c t i o n (RDF) i n t h e amorphous and i n t h e c r y s t a l l i n e phases u p t o t h e second n e a r e s t neighbours /7/. Therefore s p r e a d Ake i n t h e wave v e c t o r i s probably much l e s s t h a n t h e s e p a r a t i o n between t h e d e g e n e r a t e minima of t h e conduction band. I n t h e t h e o r e t i c a l c o n s i d e r a t i o n s /8/ it h a s been argued t h a t i m p u r i t y s t a t e s i n amorphous semiconductors resemble i n many ways t h e corresponding i m p u r i t y s t a t e s of c r y s t a l l i n e semiconductors. There is, however, a l s o an i m p o r t a n t d i f f e r e n c e : i n amorphous m a t e r i a l s i m p u r i t y s t a t e s and t h e l o c a l i z e d t a i l i n g band s t a t e s can b e d e g e n e r a t e e n e r g e t i c a l l y . T h e r e f o r e , an

-

l-

3

Z

W -

l-

&-

aIO2r

U C V) 3

G m ,

l

I

1

= - - - -

I

, I 1 t l l l l l l 1 1*:1*1*1 1 1 1 1 1 1 1 1 1 I I I I t L L

1 10 100 1000

TEMPERATURE [

K 1

Furthermore, the band extrema might have undergone a change in the amorphous phase.

With these arguments in mind we believe that the donor states in amorphous germanium behave elastically similar to those in crystals. Probably the picture of four-fold degenerate donor states in'a-Ge(H,P) may be visualized and a process like valley orbit interaction and the strain fields due to disorder of amorphous network may split the impurity ground states. In the absence of information about the sym- metry of the band extreme, it may not be possible to fix the degeneracy and symmetry of the individual levels in the amorphous case. Ultrasonic wave modulates the energy separation between the split off levels and relaxation of impurity electrons between these states attenuates the wave.

In principle the ultrasonic attenuation due to

a

relaxation process is given by /10/':

where p is the mass density and vt the velocity of longitudinal sound waves of fre- quency

w / 2 ~ r .

N represents the number of relaxing centers, in our case the number of donors. D is the deformation potential, reflecting the variation of the energy splitting between the relaxing states,due to the strain of the sound wave. The factor af/a~ describes the variation of the population of the upper relaxing level with energy. Depending on the degeneracy f is given by f

=

n[n + exp(E/kBT) I-',

where n

= 1, 2 or 3 and E is the energy splitting between the relaxing centers (in

the crystal

E = 2.8

meV). The relaxation time

T

refelcting the transition probabi- lity of the elctron can in principle also be written down, but the deformation potential for this process is not known. Furthermore the randomness oi the amor- phous structure leads to a distribution of not only the level splitting E but also of the coupling parameters. Therefore no quantitative fit of the experimental data can be carried out so far.

It is surprising that at high phosphorous concentration (sample C) the strong absorption due to the relaxation effect is suppressed. The reason lies probably in the fact that with increasing impurity concentration the wave functions of neigh- bouring impurities overlap more and more until finally an impurity band is formed.

In the latter case we cannot speak of separate states and a relaxation process in the original sense is not possible anymore. At higher temperatures

(T

2 30 K), the structural relaxation of hydrogen atoms attached to the phosphorous impurities may contribute to ultrasonic absorption like in a-Ge(H), but no direct evidence for such a process is known so far /5/.

In summary, we have studied the interaction of ultrasonic surface phonons with the donor impurities in a-Ge(H,P) down to 0.45 K. Strong influence of the impurity concentration on the ultrasonic attenuation has been observed. Presence of a broad absorption peak at about

18

K followed by a strong absorption continuing to higher temperatures is revealed in films of lower donor concentrations. At higher donor concentrations this attenuation peak is suppressed. Instead, a steeply rising attenuation at higher temperatures appears. We propose that the donor ground state in a-Ge(H,P) may be thought to have a degeneracy like in c-Ge(H,P) due to the multi- valley nature of the conduction band. A process like valley orbit interaction and disorder induced internal strain fields may lift the degeneracy of the donor ground state. This process becomes inactive as soon as an impurity band is formed.

We wish to thank M. Altarelli and K. Dransfeld for helpful discussions,

M. Bulst (Siemens, Mhchen) for supply of LiNb03 samples, H.-U. Habermeier and

R. Gibis for technical assistance in preparation of films. One of us (KLB) is

grateful to the Alexander von Humboldt-Foundation for the award of a Humboldt

fellowship.

JOURNAL

DE PHYSIQUE

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