EXCITON TRANSFER IN GaASxP1-x : N

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EXCITON TRANSFER IN GaASxP1-x : N

D. Gershoni, A. Ron, E. Cohen

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C7, supplément au n°10, Tome 46, octobre 1985 page C7-203

EXCITON TRANSFER IN 6

a

A

s

P

n

:N

x 1-x

D. Gershoni, A. Ron and E. Cohen

Solid State Institute, Technion-Israel Institute of Technology, 32000 Haifa, IsvaeI

Résumé - Les mécanismes de transfert à basse température entre les états localisés sont étudiés par l'excitation sélective à l'intérieur de la bande non uniformément élargie des excitons liés aux impuretés azote dans

GaAsxP1_x:N. Le mécanisme de transfert dominant est celui d'un tunneling

par phonons acoustiques. Le coefficient de transfert est bien plus rapide que le coefficient de recombinaison radiative. Les formes des raies observées par excitation sélective, peut s'expliquer par la dépendance en énergie de l'interaction entre exciton et phonons, et par l'existence d'un "état final" d'où 1'exciton ne peut effectuer un effet tunnel. Ces états sont directement observés par 1' émission depuis la ligne spectrale la plus basse B1 (J=2). Cette interprétation est confirmée par la levée de dégénérescence Zeeman quand un champ magnétique est appliqué".

Abstract - Selective excitation within the inhomogeneously broadened band of excitons bound to nitrogen impurities in GaAsxP^_x:N is used to study the low temperature transfer between localized states. The dominant transfer mechanism is by acoustic phonon-assisted tunneling. The transfer rate is much faster than the radiative recombination rate. The lineshapes observed under selective excitation can be explained by the energy dependence of the exciton-phonon interaction and the existence of "terminal states" from which the exciton cannot tunnel. These states are directly observed via the emission from the lower B'-line (J=2). This interpretation is further con-firmed by its Zeeman splitting in an applied magnetic field.

I - INTRODUCTION

The study of exciton states in mixed III-V semiconductors was directed mainly at excitons bound to impurities. Of particular interest is the system of excitons bound to nitrogen impurities in GaAsxP1_x and GaxIn1_xP /1-3/. The emerging

physical picture for GaAsxP1_x:N is as follows: The band of excitons bound to

nitrogen (Nx) is inhomogeneously broadened by the random potential fluctuations

which arise from the distribution of microscopic environments of the binding N cen-ters. Exciton transfer takes place between these nitrogen sites. Kash / 4 / has shown that the Nx luminescence lineshape under above-gap cw excitation can be

interpreted as due to exciton recombination at "terminal states", namely, nitrogen sites from which no transfer can take place. A similar idea has previously been pro-posed to account for the undulation spectral shape in GaP doubly doped with N and Zn acceptors /S/. Time resolved spectroscopy /6,7/ indicated that transfer times within the Nx band range around 1 nsec. In this work we provide a spectroscopic

proof to the existence of terminal states within the Nx band and show that the

transfer process between nitrogen sites is determined mainly by the exciton inter-action with long wavelength acoustic phonons.

JOURNAL

DE

PHYSIQUE

I 1 - EXPERIMENT

The c r y s t a l s used i n t h i s study were bulk GaAsxP1-x (x<0.35), l i g h t l y doped with nitrogen. They were grown from t h e vapor phase and had t h e form of t h i n needles o r parallelepipeds with well develo ed f a c e s . Luminescence s p e c t r a were e x c i t e d with a dye l a s e r ( l i n e width o f -0.12

f)

which operated e i t h e r cw o r pulsed, using a c a v i t y dumper (pulse width of 7 n s e c ) . The absorption and emission spectraowere measured with a double monochromator having a s p e c t r a l r e s o l u t i o n o f -0.05 A. A l l s p e c t r a r e p o r t e d h e r e were taken with t h e samples immersed i n l i q u i d helium.

PHOTON ENERGY (eV)

Fig. 1 - Absorption and luminescence s p e c t r a o f a nitrogen-doped GaAso.02 sample. E x c i t a t i o n i s a t 2.35 eV.

Fig. 1 shows t h e absorption and luminescence s p e c t r a o f a GaAsO 02 PO 98:N sample. The a b s o r p t i o n spectrum shows t h e Nx band r i d i n g on a smeared hand till. The luminescence spectrum c o n s i s t s o f t h e Nx band and i t s phonon sidebands. Note t h a t t h e emission phonon sidebands a r e a l s o seen i n t h e absorption spectrum. Fig. 2 shows s e v e r a l emission s p e c t r a obtained under s e l e c t i v e e x c i t a t i o n within t h e Nx

band. The n i t r o g e n concentration i n sample #278 i s about 100 times h i g h e r than t h a t i n sample #549 (which was undoped) and contained only background i m p u r i t i e s . The e n e r g i e s o f t h e e x c i t i n g l a s e r l i n e s a r e marked i n t h e f i g u r e s . In Fig. 3, high r e s o l u t i o n luminescence s p e c t r a of t h e region j u s t below t h e e x c i t i n g l a s e r l i n e s , a r e shown. These were taken f o r nitrogen-doped samples with x = 0.98 and x = 0.65. The s p l i t t i n g o f t h e s h a r p B t - l i n e under an a p p l i e d magnetic f i e l d of 28 kG i s shown i n F i g s . 3a and 3c. S i m i l a r r e s u l t s were observed f o r s e v e r a l samples-with 0.65

s

x I 0.99.

I11

-

DISCUSSION

The luminescence s p e c t r a which a r e s e l e c t i v e l y e x c i t e d within t h e Nx band c o n s i s t of s e v e r a l bands: a r e l a t i v e l y narrow band (denoted FF i n Fig. 2) peaking

Fig. 2

-

S e l e c t i v e l y - e x c i t e d luminescence of a nitrogen-doped (a,b) and undoped (c,d) GaAsOaoZ Po.gs samples. Arrows mark t h e e x c i t i n g l a s e r e n e r g i e s .

Fig. 3

-

Sharp l i n e s t r u c t u r e of t h e s e l e c t i v e l y e x c i t e d luminescence spectrum j u s t below t h e l a s e r l i n e f o r two compositions of GaAsx PI-,. The Zeeman s p l i t t i n g i s shown i n a and c.

C7-206

JOURNAL

DE PHYSIQUE

observed lineshapes a r e t h e same a s those taken with cw e x c i t a t i o n (Fig. 2 ) . The r a d i a t i v e l i f e t i m e (measured a t s e v e r a l p o i n t s along t h e N, band) i s about 300 n s e c f o r x = 0.98. Therefore, it i s c l e a r t h a t t h e FF band shape i s n o t determined by a simple competition between t h e r a d i a t i v e recombination and e x c i t o n t r a n s f e r with- i n t h e Nx band. This supports t h e i d e a of terminal e x c i t o n s t a t e s /4,5/: a t T=O, t r a n s f e r processes t a k e p l a c e between l o c a l i z e d s t a t e s u n t i l t h e exciton reaches a n i t r o g e n s i t e which i s n o t connected with any o t h e r lower-energy s i t e . The e x c i t o n

i s t h u s trapped on t h e terminal s t a t e and r a d i a t i v e l y recombines t h e r e .

- G0As0.02 %98 : ( X 2 7 8 ) T=2K

-

-

LUMINESCENCE ',

-

-

.- C *

. .

B' INTENSITY = X X FF INTENSITY X I - 8 XI I 2.28 2.29 2.30 2.31 2.32 2.33

PHOTON ENERGY (eV1

Fig. 4

-

The i n t e g r a t e d luminescence i n t e n s i t y o f t h e FF band

,

B ' l i n e and t h e luminescence spectrum e x c i t e d above t h e gap. ( A l l normalize t o u n i t y a t peak).

We have d i r e c t l y measured t h e r e l a t i v e d e n s i t y o f terminal s t a t e s within t h e N, band by u t i l i z i n g t h e s p l i t t i n g of t h e e x c i t o n

1s

s t a t e i n t o J=1 and J=2 s t a t e s which a r e s e p a r a t e d by about 0.8 meV. These s t a t e s g i v e r i s e t o t h e A' and B' l i n e s i n analogy t o GaP:N /9/. S e l e c t i v e e x c i t a t i o n i n t o t h e upper J=1 s t a t e o f a given bound e x c i t o n i s followed by r a p i d t h e r m a l i z a t i o n i n t o t h e lower J = 2 s t a t e (of t h e same e x c i t o n )

.

Emission then occurs from t h e J = 2 s t a t e . Examples o f high r e s o l u t i o n s p e c t r a which allow t h e observation of t h e B ' l i n e a r e shown i n Fig. 3b and 3d. The assignment i s v e r i f i e d by observing t h e Zeeman s p l i t t i n g of t h e B ' l i n e , which, f o r x = 0.98, i s i d e n t i c a l t o t h a t of t h e B ' l i n e i n GaP:N / 9 / . The r e l a t i v e i n t e n s i t y of t h e B ' l i n e a s a f u n c t i o n of e x c i t a t i o n energy w i t h i n t h e Nx band i s shown i n Fig. 4, and i t gives t h e r e l a t i v e d e n s i t y of terminal s t a t e s . ACKNOWLEDGEMENTS

-

T h i s study was supported by t h e U.S.- I s r a e l Binational Science Foundation.

REFERENCES

/1/ For a review s e e R . J . Nelson, i n : "Excitons", E.I. Rashba and M.D. Sturge, Eds., North-Holland, 1982, p.319.

/2/ H. M a r i e t t e , J . C h e v a l l i e r and P. Leroux-Hugon, Phys. Rev.

BG,

5706 (1980). /3/ H. M a r i e t t e , S o l i d S t a t e Comm.

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1193 (1981).

/4/ J . A . Kash, Phys. Rev. B29, 7069 (1984).

/ 5 / R.A. S t r e e t and P.J. Wiesner, Phys. Rev. 8 2 , 632 (1976).

/7/ H. M a r i e t t e , J . A . Kash, D . J . Wolford and A . Marbeuf, Phys. Rev. B z , 5217 (1985).

/8/ E . Cohen and M.D. S t u r g e , Phys. Rev.

B

s

,

3828 (1982).