- Email: [email protected]
Initial photoluminescence decay rates in amorphous phosphorus
Solid State C o m m u n i c a t i o n s , P r i n t e d in G r e a t Britain.
Vol.63,No.6,
INITIAL PHOTOLUMINESCENCE
pp.481-484,
D E C A Y RATES
R T Phillips Department
of Physics,
University
W T Toner, Central
L a s e r Facility,
(Received
1987.
IN A M O R P H O U S
0 0 3 8 - 1 0 9 8 / 8 7 $3.00 + .00 P e r g a m o n J o u r n a l s Ltd.
PHOSPHORUS
and Z Sobiesierski of Exeter,
J R M Barr
E x e t e r E X 4 4QL, G r e a t B r i t a i n
and A J L a n g l e y
R u t h e r f o r d A p p l e t o n Laboratory, Oxfordshire, OXll OQX 16 F e b r u a r y
Chilton,
Didcot,
1987 b y C W McCombie)
P r e v i o u s m e a s u r e m e n t s of the p h o t o l u m i n e s c e n c e of amorphous p h o s p h o r u s h a v e n o t b e e n able to d i s t i n g u i s h b e t w e e n two models of the timed e p e n d e n t s h i f t of the e m i s s i o n e n e r g y of the early radiative recombination. This m a y arise f r o m r e c o m b i n a t i o n of carriers t r a p p e d at c h a r g e d defects or from t h e r m a l i s a t i o n of a c a r r i e r in a tail of l o c a l i s e d states. The p r e s e n t w o r k e x t e n d s the time r e s o l u t i o n of the luminescence m e a s u r e m e n t to % 5 0 ps w h i c h enables a d i s t i n c t i o n to be made b e t w e e n the two models. The i n i t i a l decay rates for luminescence in the p h o t o n e n e r g y ranges 1.45 - 1.6 and 1.45 - 1.85 e V are the same to w i t h i n %1% at 4 K, w h i c h s u p p o r t s the a s s i g n m e n t of the r e c o m b i n a t i o n to carriers t r a p p e d at o p p o s i t e l y c h a r g e d i n t r i n s i c defects. The t e m p e r a t u r e dependence of the i n i t i a l decay follows the e m p i r i c a l law v(T) = ~. + ~ e x p ( T / T ) w i t h ~l = 8.6 x 108 s -1, ~ = 3.7 x iO ~ s -I and T = ±04 K ~ A t e m p ~ r a t u r e o o d e p e n d e n t b r a n c h i n g b e t w e e n two r a d i a t i v e channels is p r o p o s e d in o r d e r to r e c o n c i l e these o b s e r v a t i o n s w i t h e a r l i e r work.
Introduction
l e a d i n g to a p r o p o r t i o n a l i t y b e t w e e n the e m i s s i o n and e x c i t a t i o n e n e r g i e s 6,7. In the case o f a-P the L E b a n d shifts w i t h t l m e e v e n in the r e g i m e in w h i c h the i n i t i a l e n e r g y is p r o p o r t i o n a l to e x c i t a t i o n energy, w h i c h a p p e a r s to e l i m i n a t e the "thermalisation gap" approach to d e s c r i p t i o n of the recombination. Nonetheless the shift of the L E b a n d c a n still be fitted, b y K a s t n e r ' s e s t i m a t e 8 o f the energy, E, o f a c a r r i e r g r o u p t h e r m a l i s i n g in a n e x p o n e n t i a l b a n d t a i l c h a r a c t e r i s e d b y an e n e r g y s c a l e kTo: E ~ E o - 3 k T o ~ n ( ~ n v o t ) , w h e r e u o ~s o f o r d e r 1012 s -] . However, w e h a v e p r e v i o u s l y analysed t h e t e m p o r a l shift in t e r m s o f a " C o u l o m b model" in w h i c h recombination is b y radiative t u n n e l l i n g b e t w e e n c a r r i e r s t r a p p e d at o p p o s i t e l y c h a r g e d i n t r i n s i c d e f e c t s 2. In t h i s a p p r o a c h t h e t u n n e l l i n g p r o b a b i l i t y e x p ( - 2 R / R o ) (where R o is a ] o c a l i s a t i o n length) e n s u r e s t h a t t h e r e is a c o r r e l a t i o n b e t w e e n t h e s e p a r a t i o n o f c a r r i e r pairs, R, and the r a d i a t i v e time, a n d t h e r e f o r e a c o r r e l a t i o n b e t w e e n t i m e and e n e r g y v i a t h e C o u l o m b t e r m ~ I/R. In o u r s t u d y of the LE p r o c e s s c o n fined t o t h e n a n o s e c o n d t i m e range it p r o v e d v e r y d i f f i c u l t c o m p l e t e l y to e l i m i n a t e t h e r m a l i s a t i o n as an e x p l a n a t i o n for the b a n d shift. However, t h e two models offer radically different predictions for b e h a v i o u r at s h o r t e r times. The thermalisation m o d e l p r e d i c t s a s t e a d y i n c r e a s e in the m e a n e n e r g y o f l u m i n e s c e n c e as s h o r t e r t i m e s are probed, w h i l e the "Coulomb model" leads to a m a x i m u m energy characteristic of recombination of the closest p a i r s o f defects. W e a t t e m p t h e r e to r e s o l v e t h e question of the recombination channel by studying t h e l u m i n e s c e n c e d e c a y s in t h e t i m e range I 0 - I 0 0 0 ps. T h i s f u r t h e r e n a b l e s us to s t u d y t h e e f f e c t o£ t e m p e r a t u r e on the luminescence, w h i c h p r o d u c e s t h e
Amorphous phosphorus has proved to show rather c o m p l i c a t e d f e a t u r e s in its p h o t o l u m i n e s c e n c e . Two emission bands have been identified in timer e s o l v e d studiesl,2: a low e n e r g y (LE) b a n d w h i c h m o v e s f r o m about 1.35 e V to a b o u t 1.15 e V d u r i n g t h e first 200 ns a f t e r p u l s e d ( n a n o s e c o n d ) e x c i t a tion, a n d a h i g h e n e r g y (HE) b a n d w h i c h r e m a i n s n e a r 1.4 e V at all times. The LE band dominates the radiative recombination for the first few h u n d r e d n a n o s e c o n d s w h e n t h e s p e c i m e n is h e l d at a t e m p e r a t u r e b e l o w a b o u t 130K; a b o v e t h i s t e m p e r a t u r e the p h o t o l u m i n e s c e n c e a p p e a r s t o b e c h a r a c t e r istic o f t h e H E p r o c e s s at a l l times. W e s h o w e d in a n e a r l i e r p a p e r2 t h a t t h e m e a n e n e r g y of t h e L E band decreases when the energy of the exciting p h o t o n s is r e d u c e d b e l o w about 2 eV. Fasol showed p r e v i o u s l y 3 t h a t t h e HE p r o c e s s h a s s u c h a linear r e l a t i o n s h i p b e t w e e n its m e a n e n e r g y and t h e e n e r g y of excitation. T h i s f o r m of b e h a v i o u r h a s b e e n found in o t h e r a m o r p h o u s s e m i c o n d u c t o r s , and h a s been particularly carefully studied in a - S i : H 4. W h a t is i n t e r e s t i n g about t h e o b s e r v a t i o n o f s u c h a n e n e r g y - d e p e n d e n c e o f b o t h b a n d s in a-P is t h a t it s u g g e s t s that t h e t w o e m i s s i o n p r o c e s s e s s h a r e a c o m m o n r a d i a t i v e c e n t r e - just as w a s o r i g i n a l l y ~nferred (under slightly different circumstances) from optically-detected magnetic resonance 5 . One c a n d i d a t e for this m a y b e a c a r r i e r t r a p p e d in a band-tail state, s u c h t h a t for s u f f i c i e n t l y h i g h energies in the tail, energy loss by phonon e m i s s i o n leads to a t e m p o r a l s h i f t o f the l u m i n e s c e n c e band. A t low e n e r g i e s in the b a n d - t a i l (within the "thermalisation gap"), radiative recombination precedes downward hopping, thus 48]
482
D E C A Y RATES
IN A M O P H O U S
changeover from LE t o HE recombination around 130 K. P i c o s e c o n d t i m e r e s o l u t i o n allows d i r e c t observation o f the rate o f n o n - r a d i a t i v e recomb i n a t i o n w h i c h we c o m p a r e w i t h s i m i l a r o b s e r v a t i o n s o n o t h e r materials. Experimental
Technique
To achieve meaningful picosecond luminescence m e a s u r e m e n t s on an a m o r p h o u s s e m i c o n d u c t o r d e m a n d s a d i f f e r e n t a p p r o a c h f r o m that u s u a l l y a p p l i e d t o c r y s t a l l i n e materials. T h i s is a result of the e n o r m o u s range of r a d i a t i v e l i f e t i m e s found in t h e amorphous state extending from nanoseconds t h r o u g h to a p e a k in the d i s t r i b u t i o n of l i f e t i m e s at about 4 ms for a-P. In o r d e r to m a i n t a i n the s p e c i m e n in a state r e l a t i v e l y u n c h a n g e d b y t h e excitation, the e x p e r i m e n t is b e s t c a r r i e d out w i t h J l l ~ , i n a t i o n o f l o w i n t e n s i t y (to avoid f a t i g u i n g 3 ) at l o w r e p e t i t i o n rate (to avoid o v e r l a p p i n g o f e m i s s i o n from s u c c e s s i v e pulses). Our experimental a r r a n g e m e n t is i l l u s t r a t e d s c h e m a t i c a l l y in F i g u r e I. A t u n a b l e dye laser w a s s y n c h r o n o u s l y p u m p e d at
.... " ......
'
"
I
f......I......
i
I
PDA
BS ~ "
M
M
BS/
1 M
L
PHOSPHORUS
63, No.
6
s t r e a k c a m e r a t o g i v e a f i d u c i a l t i m e marker. We c a n n o t assess fully the e f f e c t o f the w e a k q u a s i c w i l l u m l n a t i o n of t h e s p e c i m e n a r i s i n g f r o m t h e unamplified pulse train, but the effects of o v e r l a p p i n g o f the p u l s e s d u r i n g t h e s t r e a k a r e s e v e r a l o r d e r s o f m a g n i t u d e b e l o w t h e lower d e t e c t i o n limit of o u r apparatus. A much more serious p r o b l e m is the p r e s e n c e o f l u m i n e s c e n c e d u r i n g t h e r e l a t i v e l y s l o w f l y - b a c k of the d e f l e c t i o n p o t e n t i a l of the s t r e a k camera, w h i c h d e m a n d s that a long b l a n k i n g p u l s e b e a p p l i e d to the grid. The length o f the b l a n k i n g p u l s e w a s v a r i e d so t h a t t h i s p r o b l e m w a s i d e n t i f i e d and eliminated. The streak camera output was passed through a two-stage image intensifier before being scanned by an optical multichannel analyser in the u s u a l way. T h e r e s p o n s e o f the s y s t e m to s c a t t e r e d laser light gives an i n d i c a t i o n o f the t i m e r e s p o n s e of t h e s y s t e m - t y p i c a l l y t h e laser p u l s e gave a p e a k w i t h F W H M - 70 ps w h e n i n t e g r a t i n g o v e r t h e same n u m b e r o f p u l s e s u s e d to o b t a i n e x p e r i m e n t a l d e c a y data. Experimental conditions were chosen so as to p r o v i d e a t e s t o f the r e c o m b i n a t i o n models. Excit a t i o n w a s at 1.968 e V - t h e p e a k of the l u m i n e s c e n c e e x c i t a t i o n spectrum. The optical bandwidth o f d e t e c t i o n w a s limited t o ~ 1.45 e V at the lowe n e r g y end b y the $25 p h o t o c a t h o d e response, and t o 1.6, 1.73 or 1.85 e V at the h i g h - e n e r g y end b y a p p r o p r i a t e "edge" filters ( w h i c h a l s o e l i m i n a t e d scattered laser light). The specimens were p o l i s h e d lumps o f h i g h p u r i t y red a-P o b t a i n e d f r o m M C P E l e c t r o n i c M a t e r i a l s Ltd, a n d w e r e p r e p a r e d b y t r a n s f o r m a t i o n o f w h i t e P4 at - 280oC. These were h e l d in h e l i u m e x c h a n g e gas c o n t r o l l e d at t e m p e r a tures b e t w e e n 4 and 300 K. Experimental data have b e e n c o r r e c t e d for n o n - l i n e a r i t y in the t i m e b a s e and for l a t e r a l v a r i a t i o n in s e n s i t i v i t y o f the d e t e c t i o n system.
F
Results Figure
Vol.
i
A s c h e m a t i c d i a g r a m of t h e apparatus. A modelocked train of p u l s e s from a s t a n d i n g - w a v e dye laser p a s s t h r o u g h a p u l s e d dye a m p l i f i e r (PDA) t o give amplified pulses superimposed on t h e m o d e locked train. T h e b e a m is d i r e c t e d b y s e v e r a l r e f l e c t o r s ( r e p r e s e n t e d h e r e b y M) t o the s a m p l e v i a a r e t r o r e f l e c t o r (R) a r r a n g e d in o r d e r t o g i v e a v a r i a b l e delay. Parts o f t h e b e a m are e x t r a c t e d w i t h b e a m s p l i t t e r s (BS) to give a t r i g g e r s i g n a l f r o m the p h o t o d i o d e P and a l s o to g i v e a f i d u c i a l marker. T h e sample is h e l d in a c r y o s t a t C a n d l u m i n e s c e n c e is f o c u s s e d w i t h lens L o n the sllt of t h e s t r e a m camera. The e n e r g y w i n d o w is s e l e c t e d b y filter F a n d the l u m i n e s c e n c e and f i d u c i a l are s u p e r i m p o s e d at the s t r e a k c a m e r a b y a glass s l i d e S.
Figure
and D i s c u s s i o n 2 shows
luminescence
transient
as
W
o
/
0 82 M H Z b y an a c t i v e l y m o d e - l o c k e d N d : Y A G laser. The pulse train f r o m the d y e laser w a s p a s s e d t h r o u g h a t h r e e - s t a g e p u l s e d d y e a m p l i f i e r %o g i v e 5 p s - l o n g p u l s e s at a r e p e t i t i o n rate of l0 HZ. T h e s e a m p l i f i e d pulses, t o g e t h e r w i t h the 82 M ~ z p u l s e train, p a s s e d t h r o u g h an o p t i c a l d e l a y b e f o r e r e a c h i n g the specimen. A b o u t 5% of t h e b e a m w a s used to illuminate a vacuum photodiode which provided a low-3 itter trigger signal for the D e l l i s t r i q u e s t r e a k camera. A small part of the b e a m w a s d i v e r t e d t h r o u g h the e n t r a n c e slit of t h e
a typical
I 1
I 2
I 3
I 4
I 5
I 6
i 7
, 8
xl0-lOs Figure
2
Photoluminescence signal obtained from a specimen h e l d at 60 K. T h e r a w d a t a are p l o t t e d in the f o r m o f c o u n t s at the o p t i c a l m u l t i c h a n n e l a n a l y s e r a n d a t i m e scale h a s b e e n a d d e d w h i c h is c o r r e c t e d for n o n l i n e a r i t y o f t h e s t r e a k ramp. T h e large i n i t i a l p e a k is t h e f i d u c i a l m a r k e r w h o s e w i d t h g i v e s a n i n d i c a t i o n of t h e t i m e resolution.
Vol.
63, No.
6
DECAY RATES
IN A M O R P H O U S P H O S P H O R U S
recorded by the optical multichannel analyser. The n o n - l i n e a r i t y of t h e t i m e b a s e r e s u l t s f r o m v a r i a t i o n o f t h e s t r e a k rate a c r o s s the field o f t h e s t r e a k tube. T h e r i s e t i m e o f t h e s i g n a l is d o m i n att~ by the characteristics of the detection system, a n d is c o n s i s t e n t w i t h a z i s e t i m e for t h e l u m i n e s c e n c e o f 50 ps o r less 9 . F i g u r e 3 d i s p l a y s data corrected for timebase non-linearity and
s i m p l e m o d e l suggests. The "Coulomb model" relies o n t h e e x i s t e n c e of a S t o k e s s h i f t t o e x p l a i n p a r t o f t h e e n e r g y d i f f e r e n c e b e t w e e n t h e e x c i t a t i o n and e m i s s i o n spectra. U n f o r t u n a t e l y o u r p r e s e n t limit o n t h e e x p e r i m e n t a l r i s e t i m e - 50 ps - is i n s u f f i cient to corroborate directly this interpretation, w h i c h i m p l i e s a v e r y r a p i d e n e r g y loss in a m u l t i phonon emission process.
•
oo
o. •
•
•••
••
•o
• ••e•
••l
•••°°•
•••0 ••
•
o• • • •
•
°°°°iiiii e.
••0• •
0•
•i
0
0•
•
•
••I 2
•0e
•
°.
•
A
•
•• i•
I•
2
i
• •
483
•.
"o
•••
3
••'''•
4
"•o.
oo• • *
•
-1
I•
•• 5
• 6 °
0
"I
I
I
2
4
6
I
8 x 1 0 -10 s
[al
Figure
0
"I
I
2
4
"i 6
I
8 X 10 -1° S
[hi
3
T h e l o g a r i t h m of t h e l u m i n e s c e n c e i n t e n s i t y v e r s u s t i m e for t h e r a n g e s of p h o t o n e n e r g y 1 . 4 5 - 1 . 8 5 e V (a) a n d 1 , 4 5 - 1 . 7 3 e V (b) at t e m p e r a t u r e s (i) 4 K, (2) 50 K, (3) 100 K, (4) 150 K, (5) 200 K and (6) 250 K. The unlabelled data correspond to the t r a i l i n g e d g e of a laser p u l s e m e a s u r e d u n d e r t h e same conditions.
d e t e c t i o n s e n s i t i v i t y in t h e f o r m Qn ( i n t e n s i t y ) v e r s u s time, o v e r that r a n g e o f t i m e for w h i c h a s i n g l e e x p o n e n t i a l d e c a y is o b s e r v e d . D a t a are plotted for the e n e r g y w i n d o w s 1 . 4 5 - 1 . 8 5 e V and 1 . 4 5 - 1 . 7 3 eV. The i n i t i a l d e c a y r a t e u(T) is the s u m o f the r a d i a t i v e and n o n - r a d i a t i v e c o m p o n e n t s the l a t t e r b e c o m i n g d o m i n a n t p r o g r e s s i v e l y as the t e m p e r a t u r e is i n c r e a s e d . ~%e logarithmic decay r a t e u(T) is seen to b e i n d e p e n d e n t of t h e s p e c t r a l w i n d o w to a level of a b o u t 1%. If t h e l u m i n e s c e n c e is m o d e l l e d b y a G a u s s i a n b a n d s h i f t i n g r i g i d l y a c c o r d i n g to the t h e r m a l i s a t i o n formula t h e n the t w o s p e c t r a l w i n d o w s w o u l d b e e x p e c t e d to lead to d e c a y rates d i f f e r i n g b y 1 0 - 2 0 % d e p e n d i n g u p o n the d e t a i l e d c h o i c e of p a r a m e t e r s a p p r o p r i a t e t o a--P. We therefore infer from these data that t h e r m a l i s a t i o n m u s t b e s u b s t a n t i a l l y less i m p o r t a n t in d e t e ~ n i n i n g the p o s i t i o n of the b a n d than the
F i g u r e 4 s h o w s t h e t e m p e r a t u r e d e p e n d e n c e of t h e i n i t i a l r a d l a t i v e rate for the w i d e r e n e r g y window, t o g e t h e r w i t h a fit to t h e d a t a of the form v(T) vl 4 V o e x p ( T / T o). T h i s e m p i r i c a l rule h a s p r e viously been found s u c c e s s f u l in d e s c r l b i n g the i n i t i a l rate in a - S i : H I0. In t h e case of a-P at 4 K v i = 8.6 x 108 S -[, u o = 3.7 x 108 s -[ and T o = 104 K. A t t e m p t i n g a fit to an e q u a t i o n of t h e f o r m v(T) = v 2 + v 3 e x p ( - E / K T ) gives u 2 = 1.4 x l• g S -i, u B = 3.5 × i0 I0 s -[ wlth E = @7 mev. Though these seem reasonable parameters we abandon t h i s m o d e l b e c a u s e of the p r o b l e ~ i d e n t i f i e d in its a p p l i c a t i o n to s i l i c o n I0, W h a t is i n t e r e s t i n g a b o u t t h e d a t a s h o w n in F i g u r e 4 is that b y - 130 K the n o n - r a d i a t l v e rate is o n l y about 3.5 times that at 4 K. Y e t m e a s u r e m e n t s of the P L s p e c t r u m in the nanosecor~ reglme s h o w that the nature of the radiatlve recombination alters r a d i c a l l y as the
D E C A Y RATES
484
IN A M O R P H O U S
6
The requirement t h a t o n e of t h e c h a r g e d d e f e c t s s h o u l d r a p i d l y t r a p an a p p r o p r i a t e c a r r i e r s e e m s likely t o b e m e t b y t h e P : centre. T h i s d e f e c t is e x t r e m e l y s e n s i t i v e t o its e n v i r o n m e n t ( R J o n e s p r i v a t e c o m m u n i c a t i o n ) e v e n t o t h e extent, perhaps, o f forming a b r o a d b a n d o v e r l a p p i n g t h e c o n d u c t i o n b a n d tail. The proportionality between excitation and e m i s s i o n e n e r g i e s w o u l d b e regarded, o n t h i s model, as a m a n i f e s t a t i o n of t h e inhouoqeneous O n t h e o t h e r hand, w e b r o a d e n i n g of the P +4 band.
4
Figure
63, No. 6
c a r r i e r t r a p p e d at o n e o f the c h a r g e d d e f e c t s and the o t h e r in a b a n d - t a i l state. The n o n - r a d i a t i v e q u e n c h i n g in the s u b - n a n o s e c o n d t i m e r a n g e d e p l e t e s subsets of b o t h radiative populations but with a l m o s t c o w ~ l e t e d e p l e t i o n of t h e LE p a i r s b y 130 K.
lo s
0
Vol.
PHOSPHORUS
I
I
100
200
r e q u i r e t h a t t h e Pd e f e c t s h o u l d lie d e e p e r i n 2 the gap and trap only a sm~ll proportion of the a v a i l a b l e holes.
300 T/K
Conclusions
4
T h e i n i t i a l d e c a y rate v o f p h o t o l u m i n e s c e n c e in t h e e n e r g y range 1 . 4 5 - 1 . 8 5 e V as a f u n c t i o n o f s a m p l e temperature, T. T h e solid line shows t h e e m p i r i c a l rule u = 8.6 × I08 + 3.7 x 188 e x p ( T / 1 0 4 K). s p e c i m e n is w a r m e d to 130 K, w i t h the l o w e r - e n e r g y t r a n s i t i o n s a b s e n t at h i g h e r temperatures. F u r t h e r more, the d e c a y in the n a n o s e c o n d reglme is c h a r a c t e r i s e d b y a lifet~ne o f the o r d e r o f I 0 - i 0 0 ns. The p i c o s e c o n d and n a n o s e c o n d d a t a c a n b e r e c o n c i l e d if the b r a n c h i n g r a t i o at p h o t o e x c i t a t i o n is t e m p e r a t u r e dependent. T h u s at l o w t e m p e r a t u r e s e l e c t r o n s and h o l e s t r a p close to one a n o t h e r and lead p r e f e r e n t i a l l y to LE recombination, w h e r e a s at h i g h e r t e m p e r a t u r e s one c a r r i e r traps r a p i d l y and the o t h e r has i n c r e a s i n g o p p o r t u n i t y to h o p a w a y as the t e m p e r a t u r e is increased. T h i s a l s o fits the asslgnment of the LE p r o c e s s to recombination b e t w e e n c a r r i e r s t r a p p e d at n e a r b y c h a r g e d d e f e c t s and o f the H E b a n d to r e c o m b i n a t i o n i n v o l v i n g one
T h i s study of t h e l u m i n e s c e n c e d e c a y in a - P f r o m 5 0 - 6 0 0 p s a p p e a r s t o b e a r out o u r e a r l i e r s u g g e s t i o n that t h e low e n e r g y P L b a n d is a s s o c i a t e d w i t h recombination of carriers trapped at pairs of oppositely charged defects. The temperature d e p e n d e n c e of t h e i n i t i a l d e c a y r a t e f o l l o w s t h e e m p i r i c a l law v(T) = v I + V o e x p ( T / T o ) d e s p i t e t h e fact that, in t h e n a n o s e c o n d time regime, the recombination channel changes to a dlfferent m e c h a n i s m above 130 K. C o n v e n t i o n a l ew4planatlons o f r e c o m b i n a t i o n m e e t w i t h d i f f i c u l t y in e x p l a i n i n g t h e s e observations, so w e p r o p o s e t h a t t h e r e is a teml~rature- dependent branching ratio between two m a i n r e c o m b i n a t i o n channels, b o t h o f w h i c h i n v o l v e c a r r i e r s t r a p p e d in a b r o a d and s h a l l o w P + band. 4
Acknowled ement
~
T h i s w o r k w a s c a r r i e d out at t h e C e n t r a l Laser Facility, R u t h e r f o r d A p p l e t o n Laboratory, and was s u p p o r t e d b y t h e SERC. ZS is g r a t e f u l t o t h e S E R C for t h e p r o v i s i o n of a studentship. W e w o u l d like t o t h a n k Dr B o b J o n e s for s e v e r a l u s e f u l d i s c u s s i o n s and for c o m m u n i c a t i n g som~ o£ his results prior to their publication.
References
1 2 3 4 5 6 "!
G Fasol, A D Y o f f e and E A Davis, J P h y s CI__55, 5851, (1982) Z S o b i e s i e r s k i and R T Phillips, Solid State C o m m u n _60, 25, ( 1986 ) G Fasol, J P h y s C18, 1729, (1985) E A w i l s o n and T P Kerwin, P h y s R e v 82_55, 5276, ( 1982 ) S D e p i n n a and B C Cavenett, J Phys _Cl6, 7063 ( 1983 ) W C Chen, B J Feldman, J Bajaj, F M T o n g and G K Wong, Solid State C o m m u n _38, 5923, (1983) F o r a d i f f e r e n t a p p r o a c h see F B o u l i t r o p and I) J Dunstan, P h y s R e v 828, 5923, (1983) and D J D u n s t a n and F Boulitrop, P h y s R e v 83__O0, 5945, ( 1984 )
8
M
Kastner,
J
Non-Crystalline
Solids
_35 and _36
807 (1980) 9
I0
Z S o b i e s i e r s k i and R T Phillips, P r o c e e d i n g s of the I n t e r n a t i o n a l C o n f e r e n c e o n N o n - C r y s t a l l i n e Semiconductors, Balatonszeplak, S e p t 1986, t o be published in J o u r n a l of Non-Crystalline Solids B A Wilson, P Hu, T M J e d ] u and J P Harbison, Phys R e v 828, 5901, (1983)
Vol.63,No.6,
INITIAL PHOTOLUMINESCENCE
pp.481-484,
D E C A Y RATES
R T Phillips Department
of Physics,
University
W T Toner, Central
L a s e r Facility,
(Received
1987.
IN A M O R P H O U S
0 0 3 8 - 1 0 9 8 / 8 7 $3.00 + .00 P e r g a m o n J o u r n a l s Ltd.
PHOSPHORUS
and Z Sobiesierski of Exeter,
J R M Barr
E x e t e r E X 4 4QL, G r e a t B r i t a i n
and A J L a n g l e y
R u t h e r f o r d A p p l e t o n Laboratory, Oxfordshire, OXll OQX 16 F e b r u a r y
Chilton,
Didcot,
1987 b y C W McCombie)
P r e v i o u s m e a s u r e m e n t s of the p h o t o l u m i n e s c e n c e of amorphous p h o s p h o r u s h a v e n o t b e e n able to d i s t i n g u i s h b e t w e e n two models of the timed e p e n d e n t s h i f t of the e m i s s i o n e n e r g y of the early radiative recombination. This m a y arise f r o m r e c o m b i n a t i o n of carriers t r a p p e d at c h a r g e d defects or from t h e r m a l i s a t i o n of a c a r r i e r in a tail of l o c a l i s e d states. The p r e s e n t w o r k e x t e n d s the time r e s o l u t i o n of the luminescence m e a s u r e m e n t to % 5 0 ps w h i c h enables a d i s t i n c t i o n to be made b e t w e e n the two models. The i n i t i a l decay rates for luminescence in the p h o t o n e n e r g y ranges 1.45 - 1.6 and 1.45 - 1.85 e V are the same to w i t h i n %1% at 4 K, w h i c h s u p p o r t s the a s s i g n m e n t of the r e c o m b i n a t i o n to carriers t r a p p e d at o p p o s i t e l y c h a r g e d i n t r i n s i c defects. The t e m p e r a t u r e dependence of the i n i t i a l decay follows the e m p i r i c a l law v(T) = ~. + ~ e x p ( T / T ) w i t h ~l = 8.6 x 108 s -1, ~ = 3.7 x iO ~ s -I and T = ±04 K ~ A t e m p ~ r a t u r e o o d e p e n d e n t b r a n c h i n g b e t w e e n two r a d i a t i v e channels is p r o p o s e d in o r d e r to r e c o n c i l e these o b s e r v a t i o n s w i t h e a r l i e r work.
Introduction
l e a d i n g to a p r o p o r t i o n a l i t y b e t w e e n the e m i s s i o n and e x c i t a t i o n e n e r g i e s 6,7. In the case o f a-P the L E b a n d shifts w i t h t l m e e v e n in the r e g i m e in w h i c h the i n i t i a l e n e r g y is p r o p o r t i o n a l to e x c i t a t i o n energy, w h i c h a p p e a r s to e l i m i n a t e the "thermalisation gap" approach to d e s c r i p t i o n of the recombination. Nonetheless the shift of the L E b a n d c a n still be fitted, b y K a s t n e r ' s e s t i m a t e 8 o f the energy, E, o f a c a r r i e r g r o u p t h e r m a l i s i n g in a n e x p o n e n t i a l b a n d t a i l c h a r a c t e r i s e d b y an e n e r g y s c a l e kTo: E ~ E o - 3 k T o ~ n ( ~ n v o t ) , w h e r e u o ~s o f o r d e r 1012 s -] . However, w e h a v e p r e v i o u s l y analysed t h e t e m p o r a l shift in t e r m s o f a " C o u l o m b model" in w h i c h recombination is b y radiative t u n n e l l i n g b e t w e e n c a r r i e r s t r a p p e d at o p p o s i t e l y c h a r g e d i n t r i n s i c d e f e c t s 2. In t h i s a p p r o a c h t h e t u n n e l l i n g p r o b a b i l i t y e x p ( - 2 R / R o ) (where R o is a ] o c a l i s a t i o n length) e n s u r e s t h a t t h e r e is a c o r r e l a t i o n b e t w e e n t h e s e p a r a t i o n o f c a r r i e r pairs, R, and the r a d i a t i v e time, a n d t h e r e f o r e a c o r r e l a t i o n b e t w e e n t i m e and e n e r g y v i a t h e C o u l o m b t e r m ~ I/R. In o u r s t u d y of the LE p r o c e s s c o n fined t o t h e n a n o s e c o n d t i m e range it p r o v e d v e r y d i f f i c u l t c o m p l e t e l y to e l i m i n a t e t h e r m a l i s a t i o n as an e x p l a n a t i o n for the b a n d shift. However, t h e two models offer radically different predictions for b e h a v i o u r at s h o r t e r times. The thermalisation m o d e l p r e d i c t s a s t e a d y i n c r e a s e in the m e a n e n e r g y o f l u m i n e s c e n c e as s h o r t e r t i m e s are probed, w h i l e the "Coulomb model" leads to a m a x i m u m energy characteristic of recombination of the closest p a i r s o f defects. W e a t t e m p t h e r e to r e s o l v e t h e question of the recombination channel by studying t h e l u m i n e s c e n c e d e c a y s in t h e t i m e range I 0 - I 0 0 0 ps. T h i s f u r t h e r e n a b l e s us to s t u d y t h e e f f e c t o£ t e m p e r a t u r e on the luminescence, w h i c h p r o d u c e s t h e
Amorphous phosphorus has proved to show rather c o m p l i c a t e d f e a t u r e s in its p h o t o l u m i n e s c e n c e . Two emission bands have been identified in timer e s o l v e d studiesl,2: a low e n e r g y (LE) b a n d w h i c h m o v e s f r o m about 1.35 e V to a b o u t 1.15 e V d u r i n g t h e first 200 ns a f t e r p u l s e d ( n a n o s e c o n d ) e x c i t a tion, a n d a h i g h e n e r g y (HE) b a n d w h i c h r e m a i n s n e a r 1.4 e V at all times. The LE band dominates the radiative recombination for the first few h u n d r e d n a n o s e c o n d s w h e n t h e s p e c i m e n is h e l d at a t e m p e r a t u r e b e l o w a b o u t 130K; a b o v e t h i s t e m p e r a t u r e the p h o t o l u m i n e s c e n c e a p p e a r s t o b e c h a r a c t e r istic o f t h e H E p r o c e s s at a l l times. W e s h o w e d in a n e a r l i e r p a p e r2 t h a t t h e m e a n e n e r g y of t h e L E band decreases when the energy of the exciting p h o t o n s is r e d u c e d b e l o w about 2 eV. Fasol showed p r e v i o u s l y 3 t h a t t h e HE p r o c e s s h a s s u c h a linear r e l a t i o n s h i p b e t w e e n its m e a n e n e r g y and t h e e n e r g y of excitation. T h i s f o r m of b e h a v i o u r h a s b e e n found in o t h e r a m o r p h o u s s e m i c o n d u c t o r s , and h a s been particularly carefully studied in a - S i : H 4. W h a t is i n t e r e s t i n g about t h e o b s e r v a t i o n o f s u c h a n e n e r g y - d e p e n d e n c e o f b o t h b a n d s in a-P is t h a t it s u g g e s t s that t h e t w o e m i s s i o n p r o c e s s e s s h a r e a c o m m o n r a d i a t i v e c e n t r e - just as w a s o r i g i n a l l y ~nferred (under slightly different circumstances) from optically-detected magnetic resonance 5 . One c a n d i d a t e for this m a y b e a c a r r i e r t r a p p e d in a band-tail state, s u c h t h a t for s u f f i c i e n t l y h i g h energies in the tail, energy loss by phonon e m i s s i o n leads to a t e m p o r a l s h i f t o f the l u m i n e s c e n c e band. A t low e n e r g i e s in the b a n d - t a i l (within the "thermalisation gap"), radiative recombination precedes downward hopping, thus 48]
482
D E C A Y RATES
IN A M O P H O U S
changeover from LE t o HE recombination around 130 K. P i c o s e c o n d t i m e r e s o l u t i o n allows d i r e c t observation o f the rate o f n o n - r a d i a t i v e recomb i n a t i o n w h i c h we c o m p a r e w i t h s i m i l a r o b s e r v a t i o n s o n o t h e r materials. Experimental
Technique
To achieve meaningful picosecond luminescence m e a s u r e m e n t s on an a m o r p h o u s s e m i c o n d u c t o r d e m a n d s a d i f f e r e n t a p p r o a c h f r o m that u s u a l l y a p p l i e d t o c r y s t a l l i n e materials. T h i s is a result of the e n o r m o u s range of r a d i a t i v e l i f e t i m e s found in t h e amorphous state extending from nanoseconds t h r o u g h to a p e a k in the d i s t r i b u t i o n of l i f e t i m e s at about 4 ms for a-P. In o r d e r to m a i n t a i n the s p e c i m e n in a state r e l a t i v e l y u n c h a n g e d b y t h e excitation, the e x p e r i m e n t is b e s t c a r r i e d out w i t h J l l ~ , i n a t i o n o f l o w i n t e n s i t y (to avoid f a t i g u i n g 3 ) at l o w r e p e t i t i o n rate (to avoid o v e r l a p p i n g o f e m i s s i o n from s u c c e s s i v e pulses). Our experimental a r r a n g e m e n t is i l l u s t r a t e d s c h e m a t i c a l l y in F i g u r e I. A t u n a b l e dye laser w a s s y n c h r o n o u s l y p u m p e d at
.... " ......
'
"
I
f......I......
i
I
PDA
BS ~ "
M
M
BS/
1 M
L
PHOSPHORUS
63, No.
6
s t r e a k c a m e r a t o g i v e a f i d u c i a l t i m e marker. We c a n n o t assess fully the e f f e c t o f the w e a k q u a s i c w i l l u m l n a t i o n of t h e s p e c i m e n a r i s i n g f r o m t h e unamplified pulse train, but the effects of o v e r l a p p i n g o f the p u l s e s d u r i n g t h e s t r e a k a r e s e v e r a l o r d e r s o f m a g n i t u d e b e l o w t h e lower d e t e c t i o n limit of o u r apparatus. A much more serious p r o b l e m is the p r e s e n c e o f l u m i n e s c e n c e d u r i n g t h e r e l a t i v e l y s l o w f l y - b a c k of the d e f l e c t i o n p o t e n t i a l of the s t r e a k camera, w h i c h d e m a n d s that a long b l a n k i n g p u l s e b e a p p l i e d to the grid. The length o f the b l a n k i n g p u l s e w a s v a r i e d so t h a t t h i s p r o b l e m w a s i d e n t i f i e d and eliminated. The streak camera output was passed through a two-stage image intensifier before being scanned by an optical multichannel analyser in the u s u a l way. T h e r e s p o n s e o f the s y s t e m to s c a t t e r e d laser light gives an i n d i c a t i o n o f the t i m e r e s p o n s e of t h e s y s t e m - t y p i c a l l y t h e laser p u l s e gave a p e a k w i t h F W H M - 70 ps w h e n i n t e g r a t i n g o v e r t h e same n u m b e r o f p u l s e s u s e d to o b t a i n e x p e r i m e n t a l d e c a y data. Experimental conditions were chosen so as to p r o v i d e a t e s t o f the r e c o m b i n a t i o n models. Excit a t i o n w a s at 1.968 e V - t h e p e a k of the l u m i n e s c e n c e e x c i t a t i o n spectrum. The optical bandwidth o f d e t e c t i o n w a s limited t o ~ 1.45 e V at the lowe n e r g y end b y the $25 p h o t o c a t h o d e response, and t o 1.6, 1.73 or 1.85 e V at the h i g h - e n e r g y end b y a p p r o p r i a t e "edge" filters ( w h i c h a l s o e l i m i n a t e d scattered laser light). The specimens were p o l i s h e d lumps o f h i g h p u r i t y red a-P o b t a i n e d f r o m M C P E l e c t r o n i c M a t e r i a l s Ltd, a n d w e r e p r e p a r e d b y t r a n s f o r m a t i o n o f w h i t e P4 at - 280oC. These were h e l d in h e l i u m e x c h a n g e gas c o n t r o l l e d at t e m p e r a tures b e t w e e n 4 and 300 K. Experimental data have b e e n c o r r e c t e d for n o n - l i n e a r i t y in the t i m e b a s e and for l a t e r a l v a r i a t i o n in s e n s i t i v i t y o f the d e t e c t i o n system.
F
Results Figure
Vol.
i
A s c h e m a t i c d i a g r a m of t h e apparatus. A modelocked train of p u l s e s from a s t a n d i n g - w a v e dye laser p a s s t h r o u g h a p u l s e d dye a m p l i f i e r (PDA) t o give amplified pulses superimposed on t h e m o d e locked train. T h e b e a m is d i r e c t e d b y s e v e r a l r e f l e c t o r s ( r e p r e s e n t e d h e r e b y M) t o the s a m p l e v i a a r e t r o r e f l e c t o r (R) a r r a n g e d in o r d e r t o g i v e a v a r i a b l e delay. Parts o f t h e b e a m are e x t r a c t e d w i t h b e a m s p l i t t e r s (BS) to give a t r i g g e r s i g n a l f r o m the p h o t o d i o d e P and a l s o to g i v e a f i d u c i a l marker. T h e sample is h e l d in a c r y o s t a t C a n d l u m i n e s c e n c e is f o c u s s e d w i t h lens L o n the sllt of t h e s t r e a m camera. The e n e r g y w i n d o w is s e l e c t e d b y filter F a n d the l u m i n e s c e n c e and f i d u c i a l are s u p e r i m p o s e d at the s t r e a k c a m e r a b y a glass s l i d e S.
Figure
and D i s c u s s i o n 2 shows
luminescence
transient
as
W
o
/
0 82 M H Z b y an a c t i v e l y m o d e - l o c k e d N d : Y A G laser. The pulse train f r o m the d y e laser w a s p a s s e d t h r o u g h a t h r e e - s t a g e p u l s e d d y e a m p l i f i e r %o g i v e 5 p s - l o n g p u l s e s at a r e p e t i t i o n rate of l0 HZ. T h e s e a m p l i f i e d pulses, t o g e t h e r w i t h the 82 M ~ z p u l s e train, p a s s e d t h r o u g h an o p t i c a l d e l a y b e f o r e r e a c h i n g the specimen. A b o u t 5% of t h e b e a m w a s used to illuminate a vacuum photodiode which provided a low-3 itter trigger signal for the D e l l i s t r i q u e s t r e a k camera. A small part of the b e a m w a s d i v e r t e d t h r o u g h the e n t r a n c e slit of t h e
a typical
I 1
I 2
I 3
I 4
I 5
I 6
i 7
, 8
xl0-lOs Figure
2
Photoluminescence signal obtained from a specimen h e l d at 60 K. T h e r a w d a t a are p l o t t e d in the f o r m o f c o u n t s at the o p t i c a l m u l t i c h a n n e l a n a l y s e r a n d a t i m e scale h a s b e e n a d d e d w h i c h is c o r r e c t e d for n o n l i n e a r i t y o f t h e s t r e a k ramp. T h e large i n i t i a l p e a k is t h e f i d u c i a l m a r k e r w h o s e w i d t h g i v e s a n i n d i c a t i o n of t h e t i m e resolution.
Vol.
63, No.
6
DECAY RATES
IN A M O R P H O U S P H O S P H O R U S
recorded by the optical multichannel analyser. The n o n - l i n e a r i t y of t h e t i m e b a s e r e s u l t s f r o m v a r i a t i o n o f t h e s t r e a k rate a c r o s s the field o f t h e s t r e a k tube. T h e r i s e t i m e o f t h e s i g n a l is d o m i n att~ by the characteristics of the detection system, a n d is c o n s i s t e n t w i t h a z i s e t i m e for t h e l u m i n e s c e n c e o f 50 ps o r less 9 . F i g u r e 3 d i s p l a y s data corrected for timebase non-linearity and
s i m p l e m o d e l suggests. The "Coulomb model" relies o n t h e e x i s t e n c e of a S t o k e s s h i f t t o e x p l a i n p a r t o f t h e e n e r g y d i f f e r e n c e b e t w e e n t h e e x c i t a t i o n and e m i s s i o n spectra. U n f o r t u n a t e l y o u r p r e s e n t limit o n t h e e x p e r i m e n t a l r i s e t i m e - 50 ps - is i n s u f f i cient to corroborate directly this interpretation, w h i c h i m p l i e s a v e r y r a p i d e n e r g y loss in a m u l t i phonon emission process.
•
oo
o. •
•
•••
••
•o
• ••e•
••l
•••°°•
•••0 ••
•
o• • • •
•
°°°°iiiii e.
••0• •
0•
•i
0
0•
•
•
••I 2
•0e
•
°.
•
A
•
•• i•
I•
2
i
• •
483
•.
"o
•••
3
••'''•
4
"•o.
oo• • *
•
-1
I•
•• 5
• 6 °
0
"I
I
I
2
4
6
I
8 x 1 0 -10 s
[al
Figure
0
"I
I
2
4
"i 6
I
8 X 10 -1° S
[hi
3
T h e l o g a r i t h m of t h e l u m i n e s c e n c e i n t e n s i t y v e r s u s t i m e for t h e r a n g e s of p h o t o n e n e r g y 1 . 4 5 - 1 . 8 5 e V (a) a n d 1 , 4 5 - 1 . 7 3 e V (b) at t e m p e r a t u r e s (i) 4 K, (2) 50 K, (3) 100 K, (4) 150 K, (5) 200 K and (6) 250 K. The unlabelled data correspond to the t r a i l i n g e d g e of a laser p u l s e m e a s u r e d u n d e r t h e same conditions.
d e t e c t i o n s e n s i t i v i t y in t h e f o r m Qn ( i n t e n s i t y ) v e r s u s time, o v e r that r a n g e o f t i m e for w h i c h a s i n g l e e x p o n e n t i a l d e c a y is o b s e r v e d . D a t a are plotted for the e n e r g y w i n d o w s 1 . 4 5 - 1 . 8 5 e V and 1 . 4 5 - 1 . 7 3 eV. The i n i t i a l d e c a y r a t e u(T) is the s u m o f the r a d i a t i v e and n o n - r a d i a t i v e c o m p o n e n t s the l a t t e r b e c o m i n g d o m i n a n t p r o g r e s s i v e l y as the t e m p e r a t u r e is i n c r e a s e d . ~%e logarithmic decay r a t e u(T) is seen to b e i n d e p e n d e n t of t h e s p e c t r a l w i n d o w to a level of a b o u t 1%. If t h e l u m i n e s c e n c e is m o d e l l e d b y a G a u s s i a n b a n d s h i f t i n g r i g i d l y a c c o r d i n g to the t h e r m a l i s a t i o n formula t h e n the t w o s p e c t r a l w i n d o w s w o u l d b e e x p e c t e d to lead to d e c a y rates d i f f e r i n g b y 1 0 - 2 0 % d e p e n d i n g u p o n the d e t a i l e d c h o i c e of p a r a m e t e r s a p p r o p r i a t e t o a--P. We therefore infer from these data that t h e r m a l i s a t i o n m u s t b e s u b s t a n t i a l l y less i m p o r t a n t in d e t e ~ n i n i n g the p o s i t i o n of the b a n d than the
F i g u r e 4 s h o w s t h e t e m p e r a t u r e d e p e n d e n c e of t h e i n i t i a l r a d l a t i v e rate for the w i d e r e n e r g y window, t o g e t h e r w i t h a fit to t h e d a t a of the form v(T) vl 4 V o e x p ( T / T o). T h i s e m p i r i c a l rule h a s p r e viously been found s u c c e s s f u l in d e s c r l b i n g the i n i t i a l rate in a - S i : H I0. In t h e case of a-P at 4 K v i = 8.6 x 108 S -[, u o = 3.7 x 108 s -[ and T o = 104 K. A t t e m p t i n g a fit to an e q u a t i o n of t h e f o r m v(T) = v 2 + v 3 e x p ( - E / K T ) gives u 2 = 1.4 x l• g S -i, u B = 3.5 × i0 I0 s -[ wlth E = @7 mev. Though these seem reasonable parameters we abandon t h i s m o d e l b e c a u s e of the p r o b l e ~ i d e n t i f i e d in its a p p l i c a t i o n to s i l i c o n I0, W h a t is i n t e r e s t i n g a b o u t t h e d a t a s h o w n in F i g u r e 4 is that b y - 130 K the n o n - r a d i a t l v e rate is o n l y about 3.5 times that at 4 K. Y e t m e a s u r e m e n t s of the P L s p e c t r u m in the nanosecor~ reglme s h o w that the nature of the radiatlve recombination alters r a d i c a l l y as the
D E C A Y RATES
484
IN A M O R P H O U S
6
The requirement t h a t o n e of t h e c h a r g e d d e f e c t s s h o u l d r a p i d l y t r a p an a p p r o p r i a t e c a r r i e r s e e m s likely t o b e m e t b y t h e P : centre. T h i s d e f e c t is e x t r e m e l y s e n s i t i v e t o its e n v i r o n m e n t ( R J o n e s p r i v a t e c o m m u n i c a t i o n ) e v e n t o t h e extent, perhaps, o f forming a b r o a d b a n d o v e r l a p p i n g t h e c o n d u c t i o n b a n d tail. The proportionality between excitation and e m i s s i o n e n e r g i e s w o u l d b e regarded, o n t h i s model, as a m a n i f e s t a t i o n of t h e inhouoqeneous O n t h e o t h e r hand, w e b r o a d e n i n g of the P +4 band.
4
Figure
63, No. 6
c a r r i e r t r a p p e d at o n e o f the c h a r g e d d e f e c t s and the o t h e r in a b a n d - t a i l state. The n o n - r a d i a t i v e q u e n c h i n g in the s u b - n a n o s e c o n d t i m e r a n g e d e p l e t e s subsets of b o t h radiative populations but with a l m o s t c o w ~ l e t e d e p l e t i o n of t h e LE p a i r s b y 130 K.
lo s
0
Vol.
PHOSPHORUS
I
I
100
200
r e q u i r e t h a t t h e Pd e f e c t s h o u l d lie d e e p e r i n 2 the gap and trap only a sm~ll proportion of the a v a i l a b l e holes.
300 T/K
Conclusions
4
T h e i n i t i a l d e c a y rate v o f p h o t o l u m i n e s c e n c e in t h e e n e r g y range 1 . 4 5 - 1 . 8 5 e V as a f u n c t i o n o f s a m p l e temperature, T. T h e solid line shows t h e e m p i r i c a l rule u = 8.6 × I08 + 3.7 x 188 e x p ( T / 1 0 4 K). s p e c i m e n is w a r m e d to 130 K, w i t h the l o w e r - e n e r g y t r a n s i t i o n s a b s e n t at h i g h e r temperatures. F u r t h e r more, the d e c a y in the n a n o s e c o n d reglme is c h a r a c t e r i s e d b y a lifet~ne o f the o r d e r o f I 0 - i 0 0 ns. The p i c o s e c o n d and n a n o s e c o n d d a t a c a n b e r e c o n c i l e d if the b r a n c h i n g r a t i o at p h o t o e x c i t a t i o n is t e m p e r a t u r e dependent. T h u s at l o w t e m p e r a t u r e s e l e c t r o n s and h o l e s t r a p close to one a n o t h e r and lead p r e f e r e n t i a l l y to LE recombination, w h e r e a s at h i g h e r t e m p e r a t u r e s one c a r r i e r traps r a p i d l y and the o t h e r has i n c r e a s i n g o p p o r t u n i t y to h o p a w a y as the t e m p e r a t u r e is increased. T h i s a l s o fits the asslgnment of the LE p r o c e s s to recombination b e t w e e n c a r r i e r s t r a p p e d at n e a r b y c h a r g e d d e f e c t s and o f the H E b a n d to r e c o m b i n a t i o n i n v o l v i n g one
T h i s study of t h e l u m i n e s c e n c e d e c a y in a - P f r o m 5 0 - 6 0 0 p s a p p e a r s t o b e a r out o u r e a r l i e r s u g g e s t i o n that t h e low e n e r g y P L b a n d is a s s o c i a t e d w i t h recombination of carriers trapped at pairs of oppositely charged defects. The temperature d e p e n d e n c e of t h e i n i t i a l d e c a y r a t e f o l l o w s t h e e m p i r i c a l law v(T) = v I + V o e x p ( T / T o ) d e s p i t e t h e fact that, in t h e n a n o s e c o n d time regime, the recombination channel changes to a dlfferent m e c h a n i s m above 130 K. C o n v e n t i o n a l ew4planatlons o f r e c o m b i n a t i o n m e e t w i t h d i f f i c u l t y in e x p l a i n i n g t h e s e observations, so w e p r o p o s e t h a t t h e r e is a teml~rature- dependent branching ratio between two m a i n r e c o m b i n a t i o n channels, b o t h o f w h i c h i n v o l v e c a r r i e r s t r a p p e d in a b r o a d and s h a l l o w P + band. 4
Acknowled ement
~
T h i s w o r k w a s c a r r i e d out at t h e C e n t r a l Laser Facility, R u t h e r f o r d A p p l e t o n Laboratory, and was s u p p o r t e d b y t h e SERC. ZS is g r a t e f u l t o t h e S E R C for t h e p r o v i s i o n of a studentship. W e w o u l d like t o t h a n k Dr B o b J o n e s for s e v e r a l u s e f u l d i s c u s s i o n s and for c o m m u n i c a t i n g som~ o£ his results prior to their publication.
References
1 2 3 4 5 6 "!
G Fasol, A D Y o f f e and E A Davis, J P h y s CI__55, 5851, (1982) Z S o b i e s i e r s k i and R T Phillips, Solid State C o m m u n _60, 25, ( 1986 ) G Fasol, J P h y s C18, 1729, (1985) E A w i l s o n and T P Kerwin, P h y s R e v 82_55, 5276, ( 1982 ) S D e p i n n a and B C Cavenett, J Phys _Cl6, 7063 ( 1983 ) W C Chen, B J Feldman, J Bajaj, F M T o n g and G K Wong, Solid State C o m m u n _38, 5923, (1983) F o r a d i f f e r e n t a p p r o a c h see F B o u l i t r o p and I) J Dunstan, P h y s R e v 828, 5923, (1983) and D J D u n s t a n and F Boulitrop, P h y s R e v 83__O0, 5945, ( 1984 )
8
M
Kastner,
J
Non-Crystalline
Solids
_35 and _36
807 (1980) 9
I0
Z S o b i e s i e r s k i and R T Phillips, P r o c e e d i n g s of the I n t e r n a t i o n a l C o n f e r e n c e o n N o n - C r y s t a l l i n e Semiconductors, Balatonszeplak, S e p t 1986, t o be published in J o u r n a l of Non-Crystalline Solids B A Wilson, P Hu, T M J e d ] u and J P Harbison, Phys R e v 828, 5901, (1983)