Decay of the red Cu-S pair luminescence in GaP

Solid State Co~nunications, Printed in Great Britain.

DECAY

Vol.49,No.8,

OF

THE

RED

pp.761-763,

Cu-S

PAIR

1984.

0038-]098/84 $3.00 + .00 Pergamon Press Ltd.

K. Moser, R. Baumgartner,

and W. Prettl

Institut f~r Angewandte Physik, Universit~t Regensburg, (Received 12 August,

I N GaP

LUMINESCENCE

8400 Regensburg, W.Germany

1983 by E. Burstein)

The red Cu-S pair luminescence in GaP has been studied by time resolved spectroscopy at several photon energies between 1.39 and 1.77 eV for times shorter than 40 ~s. Superimposed on a long lasting luminescence a fast decay was observed showing the typical behaviour of donor-aceeptor (DA) pair recombination. The transition probability W m was estimated to be 106 s -l, being of the same order of magnitude as that of the other known DA pairs in GaP.

Introduction

2.336 eV photon energies. As shown in Fig. 1 the spectrum excited at 2.336 eV consists of a broad band with peak intensity at 1.65 eV which is identified as emission due to Cu-S pair re-

Donor-acceptor (DA) radiative pair recombination has been extensively investigated for shallow impurities in GaP. Comparably little has been achieved, however, about DA pairs involving deep levels 1,2. In part this is certainly due to the strong phonon coupling which prevents the observation of any electronic structure (zero phonon line) in the luminescence spectra 3-5 Another complication of deep center recombination is the vast amount of nonradiative transitions 6. The kinetics of shallow donor-Cu pair emission and the infrared quenching of luminescence have been previously investigated on a time scale up to several minutes 7. In the present paper we report on the decay of Cu-S DA luminescence for times shorter than 40 ~s and compare the results with the recombination of other donor-acceptor pairs like Zn-S and Zn-O 2

solid line: excitotion of Z336 eV broken line:excitotion ot 1.648 eV ~_~ c~ ~-~ z L~ z

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Experimental

14.

The experiments were performed on commercially available GaP-crystals supplied by Wacker-Chemietronic. Measurements of the optical absorption and of the photoluminescence revealed the presence of the following impurities with approximate concentrations given in brackets: zinc (<2-1017 cm-3), sulphur (>1017 cm-3), oxygen (NIoI7 cm-3), and copper (>1016 cm-3). The samples were p-conducting having room temperature carrier concentrations not larger than 1017 cm -3. The crystals were immersed in liquid helium at T = 1.6 K. Interband transitions were excited at a quantum energy of 2.408 eV by 3 ns long pulses of a dye laser. The power in the laser beam of 1 mm 2 cross section was varied by calibrated filters up to 1OO ~J per pulse. The photoluminescence was detected by a photomultiplier after passing through a double grating monochromator. The signals were stored and averaged over typically 104 pulses by a transient recorder.

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PHOTON ENERGY (eV) Fig. I. Photoluminescence spectrum for cw excitation of the GaP crystal at 4.2 K. The arrows indicate the detection quantum energies of the transient measurements. The structure around 2.2 eV is caused by Zn-S DA pair decay (zero phonon line and phonon replicas). The energy shift of the peaks is due to different excitation powers. combination in a~reement with results of Monemar et al. 5,u . The Cu level involved in this transition is the so called "A-level" (0.52 eV above the valence band) whose physical nature is still not quite clear. On the low energy side a weak structure occurs which can be attributed to Zn-O pair recombinations 2 This structure becomes m u c h stronger compared to the Cu-S emission, if an excitation photon energy of 1.648 eV is applied (see Fig. i) because the oxygen level involved is directly populated.

Besides these transient measurements the cw luminescence spectrum was determined by exciting with a Kr+-laser at 1.648 eV and at

Results The decay of the time resolved photolumi-

76l

762

DECAY OF THE RED Cu-S P A I R L U M I N E S C E N C E

n e s c e n c e m e a s u r e d for v a r i o u s p o w e r s at d i f f e r e n t d e t e c t i o n h9 d r a n g i n g from 1.393 eV to p l a y e d in Figs. 2 and 3 on a me various detection quant~

e x c i t a t i o n pulse ~oton energies 1.771 eV is disl o g a r i t h m i c scale. e n e r g i e s are in-

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Vol.

49, No.

8

103 5

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-----5

2f

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Fig. 2. P h o t o l u m i n e s c e n c e decay curves for det e c t i o n q u a n t u m e n e r g i e s hu d = 1.610 and 1 . 7 7 1 e V and d i f f e r e n t e x c i t a t i o n p u l s e powers Ep (arb. units). The v e r t i c a l p o s i t i o n is arbitrary. d i c a t e d in Fig. i by arrows. The d e c a y curves m e a s u r e d at the h i g h energy side of the Cu-S b a n d (h~ d = 1.610 eV and 1.771 eV) are p l o t t e d in Fig.2 and s h o w that for decay times smaller t h a n 5 to iO Us after e x c i t a t i o n the decay is faster w h e n the e x c i t i n g pulse p o w e r is lower. For longer times, however, a slower c o m p o n e n t b e c o m e s a p p a r e n t w h i c h d e t e r m i n e s the luminescence after a b o u t 20 Us. The decay of the e m i s ~ o n on the low energy side of the b a n d is shown in Fig. 3. A g a i n there are two characteristic transients, the slower one b e c o m i n g m o r e i m p o r t a n t after 2 to 7 ~s. O n r e d u c i n g the exc i t a t i o n p o w e r the d e c a y at hv d = 1.476 eV behaves s i m i l a r l y to that on the h i g h e n e r g y side of the b a n d shown in Fig. 2. Discussion The e x p e r i m e n t a l results show that an exp o n e n t i a l fit is not a p p r o p r i a t e to d e s c r i b e the o b s e r v e d time d e p e n d e n c e of the luminescence. However, an e x p o n e n t i a l d e c a y time m a y r o u g h l y be e s t i m a t e d for the fast d e c a y i n g component y i e l d i n g a time c o n s t a n t of about iO ~s. This is in o r d e r of m a g n i t u d e a g r e e m e n t w i t h the m a j o r i t y c a r r i e r lifetime m e a s u r e d by

Fig. 3. The same as Fig. 2 but for h~ d = 1.393, 1.425, 1.452 and 1.476 eV. T h e two d i f f e r e n t decay c o m p o n e n t s are c l e a r l y visible. p h o t o c o n d u c t i v i t y in G a P : C u 9 and the r e l a x a t i o n time of the i n t e r b a n d e x c i t a t i o n i n d u c e d absorption w h i c h was a t t r i b u t e d to a t r a n s i e n t p o p u l a tion of Cu levels i0 The decay curves at short times for p h o t o n e n e r g i e s at the h i g h e n e r g y side of the luminescence band, h~ d = 1.610 and 1.771 eV, are similar to those of s h a l l o w D A pairs w h e n donor and a c c e p t o r c o n c e n t r a t i o n s are very u n e q u a l and the i m p u r i t y levels are s a t u r a t e d w i t h charge c a r r i e r s i. The time d e p e n d e n c e can be des c r i b e d b y a p o w e r law t -n w i t h n of the order of I. The s i m i l a r i t y to s h a l l o w i m p u r i t y p a i r r e c o m b i n a t i o n m u s t be due to the fact that the d e c a y k i n e t i c s d e p e n d s on the w a v e function of o n l y one impurity, in this case the s h a l l o w Sdonors, and is i n d e p e n d e n t of the u n k n o w n w a v e f u n c t i o n of the d e e p Cu-level. The decay curves b e h a v e in the same w a y for b o t h p h o t o n e n e r g i e s b e c a u s e the l u m i n e s c e n c e consists at the lower q u a n t u m energy solely of p h o n o n replicas as has b e e n shown in ref.3-5. There are also at the h i g h e r p h o t o n energy strong p h o n o n contributions so that the e m i s s i o n is m o s t p r o b a b l y due to DA p a i r s of all distances. By c o m p a r i n g the o b s e r v e d fast decay w i t h e x p e r i m e n t a l and theoretical results of A.T. r i n k et al. 2 we find the t r a n s i t i o n p r o b a b i l i t y W m = W(R-gD) for the pair d i s t a n c e R e x t r a p o l a t e d to zero to be about 106 s -I. A more accurate d e t e r m i n a t i o n of W m is not p o s s i b l e b e c a u s e o n l y one d e c a d e of the fast c o m p o n e n t could be o b s e r v e d b e f o r e

Vol. 49, No. 8

DECAY OF THE RED Cu-S PAIR LUMINESCENCE IN GaP

the luminescence was dominated by the slow decay. The transition probability is of the same magnitude as that of other DA pairs in GaP 2 There is no clear explanation for the slowly decaying component of the emission. We can exclude the Zn-O complex, because its luminescence decays very fast and is not observable at 1.610 eV II. This long time component may originate from very distant Cu-S pairs, which decay very slowly. At low intensities this luminescence can be observed several minutes after the excitation 7. This might indicate that the decay mechanism is somewhat different from that of other DA pairs. Other possible sources of the long time component could be the penetration of carriers into the bulk of the crystal, luminescence from a different impurity or interference from the slower Zn-S pair decay, i.e. Zn-S and Cu-S pairs not being isolated. Also the observation that the decay becomes faster on reducing the exciting pulse laser energy cannot be definitely explained at present. This result is in contradiction to theory and other DA recombination measurements 1,2. Again a larger penetration of carriers into the crystal at higher excitation energies

might be a possible reason for this effect. On the other hand, negatively charged Zn acceptors, whose concentration is much larger than that of Cu, could influence the Cu-S recombination by the effect of stray coulomb fields. Below about hv d = 1.5 eV the contribution of Zn-O pairs to the luminescence rises with decreasing photon energy and may even exceed that of the Cu-S pairs. Therefore we will not discuss these curves in detail. The transient behaviour, h o w e v e ~ is similar to that at higher photon energies. Again the decay becomes faster when the exciting pulse energy is reduced and a long time component of unclear origin is present in the decay curves. In conclusion we have measured for the first time the decay of the Cu-S pair luminescence for short times. The experimental results show that with the exception of a slow decay component the decay kinetics behaves similarly to that of other identified DA pairs in GaP, even the decay probability is within one order of magnitude the same. Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.

References i. D.G. Thomas, J.J. Hopfield, W.M. Augustyniak, Phys.Rev. 140, A202 (1965).

6. C.H. Henry, D.V. Lang, Phys.Rev. B 15, 989 (1977).

2. A.T. Vink, R.L.A. van der Heijden, A.C. van Amstel, J.Luminescence 2, 180 (1974).

7. H.G. Grimmeiss, B. Monemar, phys.stat.sol. (a) 19, 505 (1973). 8. B. Monemar, J.Luminescence 5, 472 (1972).

3. B. Monemar, L. Samuelson, Phys.Rev. B 18, 809 (1978). 4. L. Samuelson, B. Monemar, Phys.Rev. B 18, 830 (1978). 5. H.G. Grimmeiss, B. Monemar, L. Samuelson, Solid State Electr. 21, 1505 (1978).

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9. R.G. Schulze, P°E. Petersen, J.Appl.Phys. 45, 5307 (1974). iO. K. Moser, R. Baumgartner, W. Prettl, Solid State Commun. 45, 1051 (1983). 11. J.M. Dishman, I. Camlibel, Phys.Rev. B 6, 1340 (1972).